feet has covered most of the high points. I've only run a few 'goat events, but I'll add a few reasons to his list.
It's good training for running relays, especially when you have too few people to make an actual relay worthwhile---that is, if you have too few people, the relay becomes an individual race, not head-to-head.
Unlike most adventure races, there is no preplanning before the race. The race starts and the runners take off and start reading their maps, so there is a distinct advantage to people who can read the map well while moving. The skips and forks add to that advantage, because the people who are reading the map better can pick better routes and better skips, and be more aware of what the other runners are doing. There are a lot of stories where people who lost contact and/or decided to follow ended up disqualified because they didn't realize they skipped too many controls, or because the pack ran by a control and the follower didn't notice.
I understand the basic idea for the course is a longish M21 course with a lot of climb. Sometimes they're more grueling: Possum Trot IX, for example, is 14.8km with 880m climb. (But numbers like that can cause people to drop to the short course, so it's a trade-off.)
Here are some Possum Trot maps and a few of ebuckley's Absurdly Detailed Skip Analyses:
Possum Trot Archives
And here is
ebuckley's 2006 race report for my club's goat event, the BillyPig, with map links.