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

-- 
Tilman Schmidt                          E-Mail: tilman@imap.cc
Bonn, Germany
Yes, I have searched Google!