If you want an overly objective answer, speedrunners use different characters for different levels because they're faster in different ways, and I don't think Peach is used often. As a casual player IDK lol I always just played as Peach
Speedruns can be a lot of fun to watch - the better you know the game, the more fun it can be. Some runs get freaky, but a lot of games have glitchless categories or some kind of ruleset that doesn't break the game too badly. A lot of glitches are fascinating, too.