Is there any reason in merged meshes?

Greetings

I'm tormented by one question. Is there any reason to keep meshes not separated, but merged? Here's image with example

I'm bumped with several models, where every mesh is a separated object, and in the same time, i saw a lot of meshes, which are merged, and if there's any separated object, it's separated only because it's a moving part.

I know that merging meshes allowing to make a bevel between them, but is there any other, more significant profit?