Stack Overflow points also help to unlock additional features of the site, for example you can't comment on a post until you have 100 points, which unlocks that feature for you.
But most of these additional features seem to be mostly about being allowed more advanced ways to contribute to the community, e.g. editing other users' comments, locking articles etc.
None of the features (that I know of, my SO karma is nothing to brag about) make Stackoverflow more USEFUL to the high-karma user, than to the low-karma one.
Slashdot experimenting for a while with letting high-karma users disable ads.
They didn't disable them by default -- they just started showing a checkbox in the sidebar providing the option as a thank-you for "your contributions to the site" or something like that.
I don't see the checkbox anymore (I never bothered to check it; I don't visit much nowadays), so perhaps they decided the experiment wasn't a success.
While that is the case, SO is designed to be as accessible as possible for even the most casual user. There are even stories about cookie based accounts that are ancient as far as I remember.
So the site tries very hard to require nothing to participate and as someone else already said, the 'more advanced' privileges are not useful for most people. Why would you want to edit questions in general, unless you're the OCD type or a wikipedia separatist? :)
None of the features (that I know of, my SO karma is nothing to brag about) make Stackoverflow more USEFUL to the high-karma user, than to the low-karma one.