I'm not too proud of the amount of time I've spent on a certain cookie clicker clone.
Part of it is the competitive aspect, it's rewarding to put work into a game and not just compete with your past self, but also do it faster than other people. Another side of it, which doesn't ring true in all games, but does in most, is finding new ways to break the game or play it in a way that is not intended.
It's also just a more focused and interactive way of playing a game. I never understood Goldeneye as a speed game, those runners grind out the same minute long level tens of thousands of times just to improve it by one second. I played it a while ago and after a couple of hours I finally got into it and my heart started racing when I was waiting for the in game time to show at the end of levels.
There is so much variety too. A speedrun for Skyrim is incredibly different to one for Super Mario Bros, and even the speedrun itself can be wildly different from playing the game normally.
Dragster. Barnstorming. Spy Hunter.
Half of the games I play are complete aids but they're fun in a speedrun setting.