Idk, I just always have, same with the rarity of Profile Backgrounds. Not that a lot of people do but it's just a classification incase someone wanted to see all the Common Emoticons, all the Uncommon, etc. Same with the pages like Emoticons:Hearts it's for people looking for a type of emoticon (usually to buy on Community Market). There are some people who actively try to have the "rarist" emoticons.
Also you may have noticed, may have not. But my current project has been the Category: WIP Badges cause realistically that's why most people come to this Wiki; to see an image of the badge they'll get if they collect all the cards. It's how I found this wiki in the first place.