Why Should I do Clean Boot or Start in Safe Mode.

You can go through the event log, you can check system errors, if it's booting up you can recover files and folder PRIOR to a rebuild, you can strip out dodgy drivers and replace them. Etc etc etc

Open MSCONFIG from the Start, Run menu.

Try the clean boot (no drivers) and see what happens. In addition to the above you can load drivers individually or two or three and see if you can pinpoint the bad driver that way.

Go to your manufacturer's support site, download the driver update tool and run that. That will update drivers including BIOS.

Certainly clean booting in safe mode is a great step to fix startup issues as there may not be any other choice. But to address jskfan's question directly, why is safe mode good to check performance issues?

Safe mode is a quick and easy way to check everything at the same time. If there is still a problem in safe mode, unless there is a core service problem showing in the event logs, you are most likely looking at an issue that is either hardware or requires a complete wipe/reinstall of the OS. And in my experience, at this point if safe mode doesn't help and you have a good backup of the data, replacing the whole OS is usually faster than troubleshooting.

Now as Alex said, checking logs is a good idea before and during these steps to see if the problem still occurs. However sometimes the cause of the problem doesn't show up in the event logs.

What John has suggested is the step to take AFTER trying safe mode IF safe mode fixes the problem. It helps to narrow down the problem by enabling/disabling certain types of services and all the way down to individuals.

Those are only the preliminary steps in troubleshooting.  You then start enabling things one at a time or in groups until you find the culprit.
You could also use the Windows Performance Toolkit (not for the faint of heart)
There are other tools i.e. sysinternals process monitor

Checking Event Logs after your login in safe mode or before ?

if the problem doesn't show up in safe mode then checking the event logs there is pretty much a waste of time.
What it the exact problem you are trying to solve? Have you looked at task manager or resource monitor to identify potential problem areas?

Clean Boot/Safe Mode are just STEPS to take in troubleshooting.  

It's like asking a user to make sure it is plugged in, did you turn it off and on...etc. It may not solve the issue at all, but these are just steps to take to test.  

If for example you had an infection causing your system to show pop ups on the screen, doing a Safe Mode or Clean boot can temporarily prevent the infection from running  and in which case you can use tools to remove infection.

It's also like taking medicine, sometimes you have allergies or have the flue or cold (cough, sneeze, dry itchy throat).  So sometimes taking Vitamin C, or cough/cold/flu medication.  But if you had allergies it wouldn't  necessarily resolve that issue, but you are narrowing it down.

Also it's part of the process of elimination if you are unsure of where to start on repairing or fixing a computer.  You can use this to rule out software vs hardware issues.  If the system is blue screening, maybe it is a software issue, disabling software/apps/programs from the boot up process can help determine that.  If you disabled the offending software or program then you just need to figure out which one is causing it if you had disabled a bunch or all of them.  If it didn't work, then you'd look at checking for bad ram, or bad sectors on the hard drive or overheating CPU.  Again it all helps in the process of elimination.

Your system has a problem - and you don't know what the problem is, or where to look; but you are fairly sure it's a software (OS) problem, then you using clean boot can help.  HOW?  you disable a lot of software, and programs at startup, so - if the problem is gone then - the issue is with these disabled ones
from there, you can narrow the process down by re-enabling items in groups, or 1 by 1

Did you try updating all drivers as suggested earlier?

If this system is Windows 10, it may be faster to try a Windows 10 Repair Install and Keep Everything.

Go to the Media Creation Link:
https://www.microsoft.com/en-us/software-download/windows10

For Windows 10 V1709 and prior:
Windows 10 is running, so click on the Download button (not Upgrade Button, select Open (Run) but NOT Save. Allow the program to run. Allow drivers to update. Then select Keep Everything.

For Windows 10 V1803:
If the Repair will not run as per above:
Create a USB Windows Installation key and then run Setup on the USB Key.
This will launch the Repair and proceed normally.

John, you are losing focus on the issue. Unless the Author says otherwise, I think this is a theoretical issue on why you would use safe mode. Not how to fix drivers.

The last comment of mine is a follow up IF there is a problem in the system after it has finally started. That is why I posted it.

I do not think safe mode or clean boot can be helpful to  pinpoint the problem. it can help to isolate the issue if it is caused by the drivers, Network adapters.. etc...

Then you still have to resort to the system repair or re-install.............so it is better to do it from the get go

I respectfully disagree.

You can repair or reinstall as needed.

But if you have an infection, unless you decide to back up all data and wipe the drive and start over, you may not be able to save all data.  You may still have infection, or you may have unintentionally saved the infection and put it back on when you move the data back.

And even in your comment '...help to isolate the issue if caused by drivers...'

So if you can determine it to be a driver issue?  wouldn't reinstalling the ailing drive be faster than a Windows Repair which may not even install the driver to begin with?  or a reinstall, again same thing wouldn't necessarily install the driver at all.

Safe Mode/Clean Boot are techniques used to help aid in determining a solution yes.  

Ultimately the choice is up to you.  If you want to wipe and reinstall, or repair the OS and potentially go the long route...that's up to you.

Again Safe and clean boot doesn't determine or fix everything.  It is a stepping stone into diagnosing the problem.  If you want to not diagnose the problem, then sure don't do these steps.  

You might as well buy a new computer that'll save a ton of steps.  

Also with Safe and Clean, you can prevent future issues as well.  If it was do to an App that's causing the issue and the user reinstalls app because it was not determined in the troubleshooting steps, reinstall or repair won't solve the issue.

Just some of my thoughts.

what problem do you have, or want to solve?  if you start with posting that - you'll get better help

And as Nobus says what do you want to solve?  what is the problem at hand?  When you use methods to determine what is wrong with any device, computer, car, TV, etc there are steps to take to help NARROW down the issue and to help DIAGNOSE the issue.

You will hopefully want to find out what is causing the issue so that the issue does not repeat itself.  
Everyone has their ways of fixing a computer, but fixing it right means determining the method of failure IMO.  

And sometimes you can't determine the exact error or problem that resolve the issue.  But it would certainly be better to know the cause so that the potentiality of the issue won't be repeated.

here a problem solved by it :  https://www.experts-exchange.com/questions/29117162/Windows-10-Very-Slow.html?anchorAnswerId=42677549#a42677549

Thank you al Guys!