The principle behind this--not cheesing through the game with a Pokémon that's 20-30 levels over everything you're currently encountering--is obvious, and consistent with the similarly restrictive traded-Pokémon-obedience mechanic. It does, however, seem kind of in vain when EXP candies kinda allow you to overlevel your Pokémon for minimally more effort if you so choose. At the very least, it's only the static area encounters that are especially strong, and there's supposed to be a restriction that prevents them from being shiny, which is nice.
What bothers me about SwSh's Pokémon catching system is how it makes Pokémon that are a higher level than yours harder to catch. During the main story, it's understandable, if a little frustrating when you can't catch that level 30 Pokémon with your level 29 team. But it has no business still being active after you become the Champion, IMO.