When you get into an accident with another driver and that driver is at fault, you expect him or her to pay for the damages to your vehicle and for your medical bills and lost wages. You're suffering the impact of that driver's negligence, so there's no reason you should have to pay for that mistake financially as well.
While most drivers will have insurance at average levels, some may have inadequate insurance or none at all. If you find out that the person who hit you doesn't have insurance or is under-insured, you may think you're going to have to pay out for your medical care and eat the costs of your lost time at work. That may not be the case, though.