Yeah, I'm not familiar enough with Mercurial to really say how they are similar or different, so I'm not much help there. Sorry!

I can still try to answer the staging question though:

So - once you create a commit, it's in the repo forever, so with staging, you can gradually build up a commit without doing it all at once (adding one file at a time, over time for example). You can also stage just part of a file (just a few changes), and create only the commit that you really want to go into the repo.

I suppose it's kind of like a "commit preview" that you build over time.

In practice, a lot of people do just add all their changes and then commit immediately though - so yeah, in that case a staging area doesn't make any difference :)