Some hypotheses (I suspect actual cases of meanness are more complex than any of these):

• It signals power and a lack of fear of repercussions (both to the person one is being mean to and to witnesses).

• It is efficient at achieving certain results, like making people for whom the interaction is optional go away.

• It can serve aesthetic or intellectual purposes, especially in the form of things like sarcasm.

• It is sometimes employed as a defense mechanism to prevent unwanted closeness with others.