How is rideshare insurance different from personal auto insurance?

What is this that I hear about rideshare insurance?  Isn't the whole point of Uber that I can drive on the side to make a little extra cash?  I am not in this to start a business.  

I already have a personal auto insurance like anyone else.  If this is like a rider, I suppose I can do it but what does the rideshare portion cover that I don't already have?  What's the difference?