This service is often referred to as the "system-wide proxy server".
Posix packages should install this proxy server as a system service and update the firewall rules to allow access to its IANA assigned TCP port (14500).
This service makes it possible to start and access sessions through a single authenticated port.
The start-via-proxy=yes option allows servers to start their sessions via this service so that they are registered correctly with the local seats management system (pam / logind), which should prevent early termination if their controlling shell is destroyed (ie: if the SSH session they are started from is terminated).