well it allows /sys and /proc to be mounted with read-write permissions, which then allows for example docker or systemd-nspawn to run in a container (although there are security implications with this of course, so it's recommended to run docker etc. in a VM)
for a while now there are issues/bugs with newer systemd versions (which probably won't get fixed afaik) and it can become necessary for some software to work properly in containers.