Use this cap to provide a flat even surface for pounding the dock legs in with a sledge hammer.
It is used to protect the pipe from the sledge hammer.
This part is expected to wear out eventually as it is beaten repeatedly between the dock pipe and the sledge hammer.
The cap absorbs the blow from a sledge hammer to prevent damage to the pipe.
To avoid confusion when buying pipe that fits in this, the industry standard calls the pipe size as 1.5″ schedule 40 but the O.D. is actually 1 7/8″.