What is the main action of the triceps?
To extend the forearm at the elbow and extend the arm at the shoulder.
Triceps muscle has got medial head, lateral head and long head. Long head has got the origin from the infraglenoid tubercle. That means, when you contract the triceps muscle, long head gets support from the scapula bone. That gives additional support for the muscle to do the the action of extension on the elbow joint.