Why would Spoink not be someone's favorite? While Mudkip is objectively better, some people are blinded by various ailments, and Spoink is a very good pokemon, easy to mix up with the best even if you have very good taste.
Because they can crush boulders and use their head fin as a sixth sense to see with their eyes closed. And they are cute. Furret is an excellent HM slave; in case you are unfamiliar with the term, since you used to need the HMs to get around it was useful to have a designated party pokemon who could learn four of the most used ones: Furret can do so.