Director vs Proxy

[MRob]

Hello, I understand director is like coordinated state database for the 
proxies. Is it like a special lookup resource for Dovecot proxy?

If correct, makes sense proxies and directors should be deployed in 
pairs. But few places the docs talk about maybe different number of 
directors v. proxies or putting directors on different hardware from 
proxies.

What is reason to separate director and proxy? Thank you.