When used, the Wikipedians|Wikipedians are automatically categorized so that other Wikipedians may contact them for their input. Because of this, userboxes are not unregulated free speech, and have certain restrictions.
For affirming positive knowledge
Being knowledgable or resourceful on a topic is valuable to the encyclopedia. Certain areas of knowledge are in high demand, and there may not yet be a category for Wikipedians for it.
For displaying Wikiproject membership
It's not required, but a lot of Wikipedians like to link to their projects from their user pages. Userboxes are as good as any other way.
Userboxes are not
For divisive or hateful speech
Professing antipathy is not the same as being knowledgable about a subject. The encyclopedia's category system designed to enhance the knowledge system of the encyclopedia, and not to link like-minded antagonists. Using the system in this way breeches POV license allowed on user pages by intruding upon the NPOV nature of the encyclopedia.
For art deco
While your favorite color may in fact be blue, nobody is going to as you about it. You're welcome to subst fake userboxes about your favorite colors and flowers, but they belong in the category system.