Yes nothing wrong with stock brakes on any car, but like everything, their is room for improvement and for some of us that is what we want and prefer.
For normal street use, the factory sport brakes are very good. But better pads and discs give some people more confidence allowing them to brake a little later and harder because the after-market items are better in most cases. When you need brakes in an emergency situation, better brakes will always work better.
When you want to really push your car hard, the factory brakes and factory brake fluid, will not do the job. So a major upgrade to larger discs, bigger calipers, better brake fluid is a must. Yes this is usually an expensive investment, but well worth it.
If you have the need for speed in your blood, upgraded brakes are a must.
The debate about why can go on for ever, but then so can debates about anything related to modifying any car. If you do it you understand, if you don't you don't.