Hiya, furst to comment on here, yay!
so yeah, adoptables are basically just characters/character designs artists selll. As for the consumer, they buy to keep the rights to the design, make it an official character in any work they have, commission more art of it, etc. presumably because some people are not creative enough to make their own or they want someone else's aesthetic. However, I have seen people do nothing with these "characters", so it just looks like a huge waste of money to me. To each their own, artists make money off of this practice and people seem to like it so I cant really bash anyone for it. Definitely not my thing, though.