Process groups are used for distribution of signals, and by terminals to arbitrate requests for their input: processes that have the same process group as the terminal are foreground and may read, while others will block with a signal if they attempt to read.
This call is thus used by programs such as csh(1) to create process groups in implementing job control. The Fn tcgetpgrp and Fn tcsetpgrp calls are used to get/set the process group of the control terminal.
From the St -p1003.1-88 Rationale:
4.3BSD provides a Fn getpgrp function that returns the process group ID for a specified process. Although this function is used to support job control, all known job-control shells always specify the calling process with this function. Thus, the simpler System V Fn getpgrp suffices, and the added complexity of the 4.3BSD Fn getpgrp has been omitted from POSIX.1.