I think from a listeners point of view the idea is you can go to a groups main page and set it playing track by track, so the idea I believe is that you could check out say, a tech house group, and there would only be those kind of tunes in there.
From a contributors point of view you can put your stuff into groups for your genre, some are moderated (i.e. someone vets your work belongs to the group/genre) and some are just free for all. You can contribute by joining a group and then on your track there's an 'add to group' button.
Is it worth it ? hrmmmm..... it's minimal effort really but you don't get much back either, don't think many people really use soundcloud in this way (well i don't anyway, I follow stuff I like and then use my stream). I tried submitting some tracks to groups and get the occasional random listen I think, but the downside it people use them as an easy way to spam for followers for their type of music (you know, the kind that unfollow after about a week and have 2000 people they are following). Overall really it's probably not worth the bother, but like I say - it's pretty minimal effort. The moderated groups are probably the best (as those would be the ones I'd listen to I guess), but they can take time to approve your tune etc.
I dont understand what all the hate for these groups is about?
It's a way to get your tracks out there that little bit more. Write a track, send it to 75 different groups that are asscociated with that genre and you'll get plays on your track. More people hear your work and like mentioned above, you may gain a new follower or two?
And on the flip side, your sitting there bored, want to hear some fresh drum and bass, go to one of the numerous groups dedicated to the genre and chances are, you'll find several hidden gems, and some new artists to watch out for!