It generally covers for third party injury claims. If someone hurts themselves due to negligence on the part of the organiser/site owner then they're entitled to make a claim. The waivers that most people issue are badly written and not really worth the paper they come on but they're seeking to indemnify the organiser from you hurting yourself through your own fuckwittery/the idiocy of others and trying to claim. For example, if the site owner states during the safety brief that you cannot throw a BFG overarm, then someone does and it puts you in a coma then the site organiser has sought to remove their own negligence from the equation by including it in the mandatory brief. By signing the waiver you're effectively saying that you won't sue the site for the injury which was solely down to the idiot that threw the BFG.
Organising a game during the current restrictions in and of itself could be construed as an act of negligence by the the organiser themselves.