Why are there shutdown, sync, and halt users in /etc/passwd?

 

[root@kylinos-server ~]# grep -v nologin /etc/passwd

root:x:0:0:root:/root:/bin/bash

sync:x:5:0:sync:/sbin:/bin/sync

shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown

halt:x:7:0:halt:/sbin:/sbin/halt

 

[root@kylinos-server ~]# egrep '(shut|sync|halt)' /etc/shadow

sync:*:18357:0:99999:7:::

shutdown:*:18357:0:99999:7:::

halt:*:18357:0:99999:7:::