I'm also curious which games you played with 120 fps.
On what system? With which monitor/TV connected?
Well not many because I have a GTX1060, which I admit isn't a great card for gaming at 120fps. However, I was curious what the fuzz was about so I did play some simple games like PugB, Doom, Fortnite on Forza Horizon 4 on my Msi Mag272cqr and I honestly couldn't really tell the difference between 60 and 120fps. It does give you a slight edge in competitive gaming as mouse movement seems a bit snappier, but overall it's complete overkill, especially on consoles.
4K is the same thing. On a pc monitor it does look a lot better but sitting away from a tv, it becomes a lot less noticeable, especially compared to 1440p