Umbrella insurance covers the cost of litigation (you shouldn't need to hire your own lawyer) as well as any settlement amount reached.
As another posters notes it forces the insurance company to handle scam or frivolous lawsuits as well, rather than you having to waste your time and money.
And if it really is your (or a family member's) fault it provides prompt compensation to the injured party without them needing to go through years of litigation.
I have several vehicles & young drivers on my policy, and my $2 million umbrella for under $50/month that covers the above I consider an absolute bargain.