Earlier quoted context omitted.
That was largely iframes, though they are still in use for some things like certain methods of ad delivery. frames/framesets were often about partial updates, but mostly where script wasn't wanted so AJAX (once available) wasn't an option either.
Having lived through that era, Frames (and Framesets) were mostly killed by the proliferation of PHP on cheap hosts. A lot of Frames were used for building navigation elements without a lot of copy/pasting between HTML files (headers, footers, navigation bars, etc). The alternative to frames was either copy/pasting or ancient early static site generators. When PHP became common and cheap most people moved to server-s…
You don't (and didn't) need PHP for server-side includes. Apache has had them forever. A lot of cheap hosting is LAMP stack, so has PHP available, but you don't need to use it to get SSI.