Some functions, when performed in the mod queue, result in a collapsed entry. For example, when you delete a post:
But for some actions, such as for dismissal of a flag, the entry goes away.
Having a collapsed entry is very convenient. It allows mods to go back and take care of things they might have forgotten, or to fix something they screwed up, like a bad migration.
When the entry disappears, you cannot do this. For example, if you dismiss a flag, and think better of it, unless you remember the question title or author name, you cannot fix your error. Not that I'd ever need to do this because I'm perfect, but I can see how others might dismiss a flag when they should have acted.