I've always made the maximum amount of player characters in Animal Crossing games since the GameCube game. Partially for more storage but also partially because I enjoy the interior designing aspect of the games and having more player characters also allows for more rooms to design.
While the older games maxed out at 4 player characters per game, New Horizons allows for a maximum of 8 and I happily took them up on it. The same incentives of extra storage and rooms to design still applied, but additionally, it tied in to my island's concept. My island is a melting pot of different cultures and time periods coming together in one place, so having as many player characters as possible allows for more representation and, as you can guess, even 8 isn't enough, but hey.
The 8 reasons that VanitasFan26 mentioned are also things I do too.
I do play as all of the player characters. I gave them all their own backgrounds, personalities, interests, fashion tastes, etc. so it adds some variance.
I don't play every real world day, and am more of the sort where I focus on the game for a while and then take an indefinite break (could be days, weeks, months) before eventually returning and repeating the process.