What are typical use cases of git-reset's --merge and --keep flags?

Frankly I'm not really sure about this; I've never even used the --merge and --keep modes myself. However, the release notes indicate that git reset --merge was added in git version 1.6.2, with the following note:

• git reset --merge is a new mode that works similar to the way git checkout switches branches, taking the local changes while switching to another commit.

and --keep was added in 1.7.1:

• git reset learned --keep option that lets you discard commits near the tip while preserving your local changes in a way similar to how git checkout branch does.

Then, in 1.7.9:

• git checkout -B <current branch> <elsewhere> is a more intuitive way to spell git reset --keep <elsewhere>.

which tells us that the idea behind --keep is, you have started work on some branch, and then you realize: oh, this branch should fork off from some other point (probably the tip of some other branch). For instance you might:

$ git checkout devel
$ git checkout -b fix-bug-1234

and then do some work to fix bug 1234 for the next release; but then someone says: "hey, we need bug 1234 fixed for the old release version!" So now you want fix-bug-1234 to branch off from release instead of devel. Meanwhile you haven't committed anything yet. So you:

$ git checkout -B fix-bug-1234 release

to move it to "coming off release" instead of "coming off devel". Which works in 1.7.9 or later, but in 1.7.1 through 1.7.8 you have to spell it git reset --keep.

That might explain --keep but --merge is still a bit of a mystery.