[Dovecot] The usage of dovecot-uidvalidity* files?

[Tim Chen]

After installing a new server with dovecot 1.2.4, I found there is some new
dovecot-uidvalidity* files in user's Maildir directory. However, I did
not see such files in other servers with dovecot 1.1.2 (around 2008/07).

What is function of those files?
What will happen if I delete that files?

I am curious why need to keep 2 files:
dovecot-uidvalidity
dovecot-uidvalidity.4aa4fa4c
since the content of dovecot-uidvalidity is '4aa4fa4c'
why do we need extra files?

Thanks Timo for providing the wonderful dovecot software!
Hope you don't mind my stupid questions.

Sincerely,
Tim Chen