I volunteered at Lake Mead for the NPS Park Rangers. There were two types, Law enforcement and interp. The law enforcement did just that, patroled by boat, plane, car, and 4x4 and ran the fire dept. The interp delt with vistors either guided tours, run info centers and campgrounds. I will tell you this that it is a very political game they play. A lot of Rangers volunteered while going to school for a better chance to get in. Most still did not make it and went on to do something else. Even a few who did make it just used the park service as a stepping stone to the FBI.
I did not volunteer to become a Ranger, I did it for the excitement and adventure. Like stopping a plane while in a boat, having to babysit 83 LSU sorority girls on spring break, finding a governors son who was going to kill himself before he had the chance to do it, saved a few lives, and so much more.