I'm afraid this could be a very stupid question, yet I cannot think of an explanation. Password managers (keepass, etc.) often have a button that lets you "lock" the database, so if you need a password you will have to unlock it again by entering the master password. The database could also be locked automatically after some time, or when the screen saver starts, etc.
It's clear why you should unlock the database the first time you use it after you turn on your computer, because it's encrypted. But why should you ever need to lock / unlock it again afterwards? The first thing that comes to mind is "what if an attacker accesses you computer while it's turned on and you are away", but for that matter there should be a screen locker, otherwise you are screwed anyway. It doesn't seem to prevent problems with malware or cold boot attacks either.
What threat is this trying to mitigate then? I can only think it can be useful to protect against a very low-level attacker with no skills at all, in case you leave your computer and forget to lock the screen. This kind of attacker would be able to take a look around (and read info from your unlocked password manager) but they wouldn't be able to install malware or compromise the machine in any other way. Or maybe I'm missing something.