I was wondering what the difference was between 60 fps and 144 and why it's a rule to display it at all times

Cutscenes skip way faster on 120 FPS which makes one skip possible that just backwalks out of the very first spellchallenge. Saves like 2 minutes. It also requires different angles for wallwalks (most noticeably, the one to skip the Skurge challenge gets a lot easier) The rule to display it at all time is just to prove that you dont abuse any of the benefits coming with those higher FPS and guarantee a fair and even competition.

Questions about 60 vs. 120 fps have been asked a fair few times in the forums already. I've added a new sticky post at https://www.speedrun.com/hp2pc/thread/v1vpu explaining the history and decision in more detail. A shorter explanation from a few years ago is also included in the rules for "Any% 120fps"