As an individual it's a chance to make sure you can connect with all the mods, and that the gear you spawn with is correct. That any speciality equipment is present, and any other custom values are set. That resupply is correct, and any vehicles are working as intended etc. It is the responsibility of the individual on specialist roles to be present for this, or they forgo some complaint if it doesn't pan out in the operation itself.
It's also a chance for management to make sure the mission loads in the server environment with multiple people present, and that custom mission scripts work. Should it not, we generally can fix it in time for the operation itself.