1) So you can revert the merge very easily.
2) Merging without a commit.
3) A branch on a remote server that you have a copy of locally and you want to keep track of the status of remote against the local copy (who's behind and who is ahead?).
This is my basic understanding in terms of how I use git. I like Mercurial too and don't have a horse in this race.
> git pull origin master
(fix conflicts)
> git commit
That's it.