Because it is cool
Also, extra speed is great for pretty much everything except the games, (lots of them as you say) which have fixed performance by the custom chips. Even those will benefit from an upgrade because WHDload can then eliminate much loading time.
3d games like Frontier and Interceptor feel nearly unplayable to me on a basic amiga but are super smooth with a bit of acceleration. Stuff like computer turns in civilisation is much improved too.