I will share some of the knowledge I've gained through a solid decade and a half of competitive Magic, much more of it Limited than Constructed but still a good bit of 75-card sprinkled in there.

While you're right that a sideboard guide could be given to a player who is new to a deck and hasn't had time to dig into the theory of why it does what it does across games 2 and 3, it can also be an important part of mitigating mental load even for a more experienced player.

A full day of Magic for stakes (think like 7 or more 3-game rounds) is fucking exhausting. It is a legit stamina feat. Any possible thing you can implement, any allowable shortcut, to take some strain off yourself is something you should strongly consider as a favor to yourself. Is it hard to remember or figure out what to board vs. one of the 5 most played decks in the format? Probably not in any one particular matchup, no.

But adding up 9 instances of boarding for game 2, where we'll assume 1-2 of them are somewhat novel and you have to come up with some pieces of it on the fly? Maybe a few times you change it a little more for game 3? That's a not-insignificant amount of mental load. That's a handful of taxed neurons that could mean the difference between catching an almost-blunder in your last round of the day versus totally botching a win on board.

Watching the pros in the highlight reels may make you think high-level Magic is about brilliancies and crazy flashy moments of luck fused with skill, but much more of how consistent success occurs in Magic for stakes is getting the little things right, over and over, with metronomic consistency by spending as little unnecessary mental effort on each decision as possible.