While this may be marketed on the basis of "Do it for the children," sport specifically and play in general are important mechanisms of adult social interaction. Futbol is no more a child's game than dominoes.
Tangentially related to this point, I've often wondered if this is simply a zero-sum game, and the unity you gain through supporting a particular team comes at the cost of disunity with supporters of other teams.
Sports team rivalries produce all kinds of stupidity the world over. People are routinely killed or severely injured simply because of the sports team they support.