What is the advantage of having 2 Init systems ? Mx-linux has by default sysvinit but can boot from the grub into systemD. why would a distro need this ability ?
To give the user a choice, so that they can use what they prefer to use.
MX Linux ships with systemd present but sysVinit is still the default init system by default. Thanks to the systemd-shim system, users can choose to boot installed systems whichever way they choose.