personally, i just hoard lower-value pets because i particularly like the design or heavily connect to it in some way!
my
main hoard is a bunch of omgsc bwolves from november last year, because i connected the design heavily to a specific song i like, and i had fallen in love with the design!
my other hoard is the blue and yellow bwolves of the same litter, and i don't like those designs quite as much, so that hoard is smaller
my third hoard is just random epps kittens! i want a full collection of all of the ones in the game, but i won't turn down doubles, so i've labeled it a hoard :]
and because i'm neurodivergent, i feel like some of it could also be visual stimming, because those designs make me happy and seeing so many is really nice :D
for me it really is just liking a design or connecting to it, i'm sure others have different experiences though!