Use the mirrors to keep an eye on your tires. It's one way to spot a tire fire. If you're carrying open cargo, you can use the mirrors to check it. Look for loose straps, ropes, or chains. Watch for a flapping or ballooning tarp.
Special situations require more than regular mirror checks.
You need to check your mirror to make sure no one is alongside you or about to pass you. Before you change lanes to make sure there is enough room. After you have signaled to check that no one has moved into your blind spot. Right after you start the lane change to double-check that your path is clear. After you complete the lane change.