The National Park relies on three repeaters to allow for radio communications between Marblemount and rangers throughout most of the park. Ruby is one of the three.
Where are the other two repeaters?
One is on MacGregor, and I've forgotten the other; I think Copper Mountain, to cover the N section. Since I wrote the comment, I learned there's also a new one on Finney Peak. I'm not sure whether NPS owns it or borrows it from the Forest Service when needed.
The rangers get by with 5-watt handheld portable radios that generally work line-of-sight. They'd be pretty much worthless in that lumpy country without the repeaters.