The way your car handles, your gas mileage, and vehicle safety, are all affected by the amount of tread left on your tires. If you let them wear down you increase the chances of poor handling (e.g. sliding in the rain), a flat, and even worse a blowout.
TIP: Check your tread wear once a month. A good rule of thumb with mst tires is to place a Penny inside the tread groove and if the top of Lincoln's head can be seen, the tread is worn away and the tires should be replaced.