It gives a set place / explicit command for handling:
~Kernel modules (Loading them, blacklisting them, modifying parameters)
~Basic ACPI (which you can use if you want, if you don't then just use gnome or kde with power management enabled-- they properly register themselves as the ACPI handlers and systemd respects their override)
~Location, syntax, and commands for service management / service files
Distros are gonna modify upstream for random, bullshit reasons. Should they? No. Thats why I run Arch, because I got tired of the modifications. Systemd forces them into conformity on certain things, things that have ZERO reason to be different across distros.