Encrypting a backup is really just an additional layer of security for you, all things considered. I do know that if you encrypt your backup, I believe most of your passwords to various accounts across the device are also stored in there as well (this is not true with unencrypted backups, FYI)...that function might be inviting to some as it would make the transition to a new/replacement device a little easier and less time consuming (since your accounts will still be signed in on many of the apps on the phone).
I don't ever encrypt my backups really, but I don't really have any good or legitimate reason why, I just haven't ever done it. Given how easy and effortless it is to do so though, I might start doing it from now on.