It's a wildly successful marketing tactic that supports relationships to gender, purchasing, space, and human relations that are destructive.

It emphasizes imagined difference between men and women and implies that men need some special place protected from women to be comfortable or to be themselves. It sets up and/or reinforces an adversarial gender dynamic in the home and in purchasing/marketing. It reinforces notions of men as more primitive/less refined than women. It encourages people to buy/rent homes that are larger than they need so they can create said "man cave" and then fill them with shit they don't need.