FETCH_HEAD question

I did this the other day out of mild curiosity:

$ git fetch $ git merge FETCH_HEAD

Which did something, but not something that was at all useful. It merges in the first ref listed in FETCH_HEAD. It does not appear to be an accident that it does this, as git merge has special treatment for FETCH_HEAD to generate the merge message.

Why does this behavior exist? Historical?

j.