Basically, I'll give you an example.
You crash into a exotic motor lets say a Lambo, and write it off ££££. Your insurer will go through your motor with a fine comb to find a way out of paying the claim. If they do (and they are well within their rights) you'll be liable for the damage caused. I.e be prepared to file bankruptcy. Likewise (more up my street) crash your car into a house even bigger bucks would you believe.
Then think to yourself, was it worth the extra few quid? You don't know whats round the corner (literally) is it worth the risk of that happening?