Fortnite custom settings?

I have in many YouTube videos where different graphics cards have been tested to see how much fps you would have noticed with different processors. And why do the professionals play on limited fps, which makes the fps more stable or why? And should you generally play with limited or unlimited fps?

The top picture

This is because the PC has to render less and has more power to load more frames. And I think the streamers cut the FPS to have more power for the stream