I believe it's called a "hybrid listen server" and it's role is to keep the master clock of the match running and to mediate between clients when they have too large of a discrepancy between timestamps on an event. It's supposed to give the benefits of a non player host which dictates the reality of the game whilst retaining the benefits of non centralised p2p networks which theoretically can be much faster if all players are in close proximity. Of course we all break it by doing things like running in transatlantic fireteams, using a busy badly configured WiFi router and playing using 3g and 4g phones as WiFi Hotspots.