You pretty much had it right! People just really like them and want a bunch of them for funsies.
Sometimes they're also to commemorate something (like my hoard of crow dogs; I set the maximum hoard number to be 93 in honor of Brandon Lee).
Any of my other hoards tend to have wishlist trading enabled though. I don't like the idea of accidentally artificially messing with the site's quantity of a certain pet by hoarding too aggressively, so it felt like a compromise; I can enjoy the fun of hoarding, but still avoid unbalancing the site's economy.
All in good fun, you know? :]