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

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