For people wondering why, it has mainly to with the fact that mouse polling rates are tied to FPS meaning that you still have heavy incentive to go above the FPS your monitor could handle.
This video (https://www.youtube.com/watch?v=hjWSRTYV8e0) does a really good job explaining frame latency in CSGO and why you can benefit from running at FPS significantly higher than your refresh rate, and why vertical sync doesn't solve it like it should. The game is free now, so I'd highly recommend trying it out if you're doubtful about a smoothness difference between 100 and 250-300 FPS.
The real reason is, I only really play csgo and pubg, I have X$ to spend on a CPU. Previously if you looked at them market, tier-to-tier amd vs intel - intel always was the stronger performer at the CSGO. Why spend X$ and get 250fps when you can spend the same and get 300fps?