Frames were (and to an extend still are) heavily abused for advertising and other purposes. You could, for example, hide the real URL of a page. The user would

Frames were (and to an extend still are) heavily abused for advertising and other purposes. You could, for example, hide the real URL of a page. The user would go to the page, the page would contain the frameset, and the frames would just load content from wherever. Really broke the promise of the URL bar, which would suggest that the page you're seeing belongs to the URL displayed.