Use this to provide a flat surface for pounding the dock legs in with a sledge hammer.
This is a wear part and expected to wear out after long use.
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″.