Adblock doesn't just block ads, it blocks based on a blacklist that sometimes is a little overzealous. For example, I could block images.yahoo.com, but that would block all images from Yahoo. Maybe that's fine, since I never go to Yahoo. Then a blog post I'm reading has a picture that is hosted at images.yahoo.com, and suddenly Adblock is hampering my viewing of a site that I want to see. I disable Adblock, find out what was being blocked and why, and edit the filters to fix that (or completely whitelist the site, if the ads are non-existant or non-intrusive).