On various campaign levels (Halo,
Silent Cartographer, AOTCR) you have the opportunity to get a marine to ride
shotgun, either in the Warthog or on the side of the tank. Whenever possible,
keep Sarge alive and get him to ride in the passenger seat. Which is to say, if
some other marine is sitting where you want Sarge to be, kick him out and drive
over to Sarge. If the marine is being really pesky, 'accidentally' run over him.
Why? Because Sarge carries a sniper rifle with him and he's ridiculously
accurate. Not only that, but he'll use his keen vision to spot enemies before
you do and open fire, alerting you to their presence and location.
Good ol' Sarge!