Why would I need also Dovecot in addition to Postfix?

I run my on mail server and I use Postfix/Dovecot/SquirrelMail/ThunderbirdMail. I know that Postfix is SMTP server and Dovecot is IMAP/POP3 server. Could you explain to me in simple English why Postfix isn't enough. What exactly each server does? How they interact together? Why there's no just ONE mail server that could do it all alone?
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Dave BaldwinFixer of ProblemsCommented:
Postfix is to send email, Dovecot is to receive email.  This is the 'traditional' way it has been done.  No one has chosen to write a single server for both purposes.

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