This is the small driving cap for small jobs.
It is used to protect the pipe from the sledge hammer.
There is a hole to tie a rope so it does not bounce off and land in the water. Rope should loop around the pipe being driven.
This part is expected to wear out eventually.
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″.