Imagine YouTube depending on QuickTime, Windows Media Player, RealPlayer, or Java Applets. Flash's user experience was just so much better than those options.

Imagine YouTube depending on QuickTime, Windows Media Player, RealPlayer, or Java Applets. Flash's user experience was just so much better than those options. I'd guess that YouTube couldn't have had that much success were there not such a ubiquitous (and downright good) plugin.