If you mean GNU parallel, it has way way more features than xargs -P, (and a few saner defaults).

See e.g. https://www.gnu.org/software/parallel/parallel_tutorial.html

If you mean moreutils parallel, yeah I agree it's not useful.

But that’s the point of parallel. The use case is for when you have N processors and a 10Gb NIC. Each job is CPU bound or concurrent license bound, or some jobs may take longer than others. Parallel allows you to run X jobs simultaneously to keep the CPU or licenses busy.