A robot arm on its own can only move a mounting flange through space. The end effector is what gets bolted to that flange and does the real job. Swap it and the same arm changes trade: a two-finger gripper for handling parts, a vacuum cup for cardboard boxes, a spindle for deburring, a spray head for paint.
End effectors carry their own wiring, air lines and sometimes their own sensors and small controller. Their weight counts against the robot payload, so a 10 kg rated arm fitted with a 3 kg gripper can only lift about 7 kg of part.
In a car plant one welding robot may keep a single spot-welding gun for years, while a parcel-sorting robot uses a vacuum array that can pick a padded envelope and a shoebox with the same motion.
