Installing CItrix VDA Agent on Server OS

on this EE site, I was trying to understand the layout of Xenapp/Xendesktop 7.6 in comparison with Xenapp6.5 version and previous ones.
 

I posted this question below, I got answers, but the thread was too long and the answer from Carl was clear, though I did not want to ask for citrix link for confirmation
https://www.experts-exchange.com/questions/28707905/Pubish-Applications-on-Xendesktop-XenApp-6-7.html

Well, in Xenapp 6.5 version and previous ones , we used to install Xenapp server then install applications on the Xenapp server and publish the applications to end users.

Now, as I have been told, we do that either on Xendesktop7.6 or Xenapp7.6:
1- Create Machine catalog and select Windows Server OS to install on the Machine you are delivering, install VDAgent on Windows Server you are delivering.
2- Create Delivery Group, and install Applications on the Windows Server OS you have installed on "Create Machine Catalog" step 1, then select the Active directory user groups that will have access to the published applications.

I have been told that when you install VDA agent as indicated on step 1, the server you are delivering will automatically become Xenapp server. It sounds good, but I need Citrix Link that confirms that.
Knowing that Xenapp7.6 or Xendesktop 7.6 is already installed through DVD at the first place, then why not just using the existing Xenapp7.6 or Xendesktop7.6 to install Applications on it then publish(Deliver) them to end users ?

Any clarification will be helpful

Thanks