It's a graybeard thing. 20 years ago, cp would screw up permissions, dates, ownership, symlinks, device files, etc. Different platforms would require different command-line options and then screw up different things. It was insane. Tar, on the other hand, pretty much got it right on every platform.
Nowadays cp -a works well everywhere (in my experience) so there's no need to resort to tar. It's just damage from the Unix wars.