cannot see git-fetch result in gitk

I'm still learning my way around git and trying to understand how it works. So after reading http://www.kernel.org/pub/software/scm/git/docs/howto/rebase-from-internal-branch.txt I wanted to try and understand the command sequence

git fetch origin git rebase FETCH_HEAD

by looking at what it does with gitk. But after the first command I do not see any change at all in gitk. Only after the second one do the newly fetched objects appear in the gitk display.

Simple question: why?

Thanks, Tilman