That room typically has multiple purposes: a place for men to work, to be away from women and from female sensibilities, to indulge in hobbies, or to hang out with his friends. It is, loosely, a male-only space to watch sports or play video games.
In a sense, for married men, it is a way to recreate some of the space and freedom of their bachelor days. Of course, for a woman's sake, it should at least be attractive - usually thematic in design (for his sake) depending on the man's hobbies or occupation.
Ladies, here's to your man's space...
Peace and Blessings,
Photos courtesy of HGTV.com, DIYNetwork.com