I've been there, we stopped off ther on our way home from a golfing weekend in Devon. It wasn't as big as I was expecting, but enjoyable to see.
...and jeezz I've never seen so many foreign tourists and coaches in one place, and it's in the middle of no where.
as for its purpose or use, aint realy got a clue. lol
have a look here, plenty of interesting reading
http://www.english-heritage.org.uk/server/show/nav.876
:)