Crowd control is one of the most critical and under rated mechanisms in the game. There are two types of crowd control, hard and soft. Hard usually causes a loss of control of your character (such as being feared or knocked up/down), while soft usually causes you to be unable to move, but you can still cast spells or use skills (such as being snared by Talons or Bombard).
Lets consider a NB’s fear skill. It hits 3 targets, and puts them out of the fight for between 1 and 2 seconds (one or two casts of skills). One player casting fear doesn’t do any damage or healing, though negates the activities of three opposing players for the same amount of time (or more) it took to cast the fear. Unfortunately (or fortunately depending on how you look at it), fear is not a smart targeting system, and may hit someone who is immune from cc (as they’ve just broken out of a cc effect), so continued fears within a large group of opponents will probably only take out two targets after the first few seconds of fighting, though two for one is still very good especially if you’re outnumbered!
Snares are critical to taking out groups that move tightly together. A few people casting snares will cause some opponents to get stuck for a second or two, fall behind the moving group, and lose the protections that being in said group (all balled up) provides them. They’ll be much easier to kill. A strung out group can be focused down one by one, whereas a balled group needs to be AoE’ed.
Having your NBs cast fear regularly, your DKs talons, your archers Bombard (though consider that you can’t snare someone with cc immunity using bombard), your ice staff users slows and the use of ultimates that knock up (Meteor), down (Nova) or snare (ice destro staff ultimate) in the right place can all cause your opponents to get out of position, panic, or use up their critical stamina resources getting out of them. A player with no stamina can’t block or escape, and can be killed much more easily.