There are millions of websites out there. Many of them are unique, either in small ways or in large ones. But the individual impact of any particular site on the overall Internet is generally negligible, if there's any impact at all.
Not so with the fifteen sites here. These sites changed the Internet, mostly for good, in substantial ways. Included here is everything from Geocities (which could probably be blamed entirely, either directly or indirectly, for every ugly web design "trend" that's ever been) to Wikipedia (which has made information almost universally accessible) to Google (which has changed or influenced virtually everything online).