The umbrella is for a million dollar coverage that kicks in once the regular auto coverage has been exhausted. Most coverage stops at $100,000/$250,000 of damage which is pretty easy to do. The umbrella is only $200 per year.
It sure offers a lot of extra protection for the money. For example, if you get tangled up with a big truck, many times the cargo is worth more than what regular coverage covers. And a motor home would take big bucks if you hit a diesel pusher.