Groups seem like a rather new feature to vBulletin, at least in the way they are used so far. I presume that they may plan to make a widget for the front page, but there isn't one currently. As for their usefulness. I guess it sort of works like a way to show special interests, see how many people are interested in what. I think it may be useful for development groups to show a small showcase of their exploits via photos of their team members at events or while working as a team on projects, etc. Even screenshots of their games and so on. It's not meant to replace the forums in any way, just be a separate feature on the site that's all.

Use 'em, don't use 'em, up to you.