Because of the simple design, prime lenses often have much better image quality compared to cheap zoom lenses.
When the companies design a 'kit lens', the most important factor is that it be cheap, and less important that it be light weight etc. In other words, image quality or maximum aperture are not the top priorities...so there is a lot of compromise in those areas.
So by 'replacing' your kit lens with a prime lens, you likely get better image quality and a larger maximum aperture, which has several benefits.
This leads us to high quality zoom lenses. They can be as good or better than cheap prime lenses...but they are very expensive. And past that, we have very high quality prime lenses...they are also very expensive but the image quality is outstanding.