Not at Apple and don't have any knowledge here, but I'd imagine that the UNIX test suite, ever since it began passing, has been a useful set of additional regression tests even outside the certification context.

Does anyone want to be the person that removes regression tests from active use, only to be responsible when something breaks that would have been caught by that test? Far easier to just fix your code so the test passes.

(And for many years, OS X then macOS had a reputation for being rock-solid, capable of going much longer betwen restarts, going into BSOD much less frequently than Windows would. Having a set of third-party tests certainly didn't hurt this!)