It's main tasks are exporting environment variables, partly after performing corresponding checks, and launching lxqt-session, the LXQt session manager.
It is not meant to be run by users. Rather, it is invoked as backend, e. g. by script startx on virtual terminals or display managers like SDDM or LightDM, see section EXAMPLE.
to file ~/.xinitrc. An LXQt session will then be launched by running startx.
Display managers are making use of scripts like startlxqt automatically. Information about
available desktop environments is provided by files $XDG_DATA_DIRS/xsessions/*.desktop, typically
/usr/share/xessions/*.desktop.
File lxqt.desktop provided by LXQt is stating startlxqt in key Exec while making sure the session
manager lxqt-session does exist by stating it in key TryExec.
https://www.freedesktop.org/wiki/Specifications/desktop-entry-spec/