What is systemd doing in terms of inter-Unix portability? Unix is not only Linux. Also; standardisation means not to be used all over. It means to be bound from below and above so that there is an interface that can be trusted to exist in all the adherents of that standard. For example, Linux may have send_sher_shcön(7) exposed exclusively, but we can be sure that there is send(2), for portability concerns.

edit: man numbers.