Anger usually stems from a boundary being violated. It's our instinctive evolved response for pushing back against someone who has encroached on us.

When we "vent" to a peer, it is relieving because it transfers that personal boundary to a group boundary. We feel "OK, my peer will now back me up the next time this happens."

Venting is like yelling for reinforcements when you see someone storming your corner of the castle wall.