For compatibility with older distributions such as ubuntu 18 and others.
Now /lib is a link to /usr/lib, but before they were different folders, and the systemd units were located in /lib/systemd/system.
/usr/lib/systemd/system - this folder did not exist before.
For compatibility with older distributions such as ubuntu 18 and others. Now /lib is a link to /usr/lib, but before they were different folders, and the systemd units were located in /lib/systemd/system. /usr/lib/systemd/system - this folder did not exist before.