[pilif]

I use it on my mail relay to determine whether a domain is local or not. As this check happens for every incoming RCPT TO (including all the bots checking to see whether I'm an open relay or not), I don't want to hit the database where the accounts are stored just yet.

This is the perfect case for a cdb file: the list of local domains changes very rarely, but lookups happen 100s of times per second.

Granted though, it's a very special case. Aside of this mail case, I've yet to find another real world application.