The four reasons I can think of are:
1. Appearance: you’ve got to like the way your bike looks, so why not.
2. Sound: has some merit from a safety standpoint, but also signals your whereabouts to the fuzz.
3. Cheaper than replacing original parts usually.
4. Performance increase: a good aftermarket pipe is about the simplest and cheapest form, dollar/HP of power increase.
However in lieu of no. 4 , if you want to go faster with more safety, the best monetary investment you can make is to spend on more training in the form of advanced rider courses, race schools, and track days.