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This page in a nutshell: User categories are navigation aides for Wikipedians, to be used for collaborative purposes to build the encyclopedia.
A user category is a type of category—a grouping of related pages intended as a tool for navigation[1][2]—that contains the user pages of Wikipedia users. The technical function of a user category, therefore, is to group the user pages of Wikipedia users who share one or more relevant characteristics. Given this technical function, and considering the principle that Wikipedia is an encyclopedia and not a social networking site,[3] the purpose of user categories is to aid in facilitating coordination and collaboration between users for the improvement and development of the encyclopedia.[4][5]
In addition, as with all categories,[2][4] user categories should not be used as "bottom-of-the-page" notices. If a Wikipedian wishes to have such a notice, they may edit their user page and add the notice in some other way (such as by adding text or a userbox) instead of creating a category group.
^"Categories are intended to group together pages on similar subjects." – Help:Category
^ ab"The central goal of the category system is to provide navigational links to Wikipedia pages in a hierarchy of categories which readers, knowing essential—defining—characteristics of a topic, can browse and quickly find sets of pages on topics that are defined by those characteristics." – Wikipedia:Categorization