They can be a problem since sites like this can generate heated discussions normally. A troll taking advantage of that to stir up bad feelings can disrupt relationships among the regular members.
We do our best to eliminate them. The best weapon we have is the advice Please don't feed the trolls! Not taking the bait, ignoring their taunting, will leave them with little to do but leave.
Motivation? Some people have no friends so the best they can do for attention is to cultivate enemies.