It's easier on your body.
Treadmills offer better shock absorption, resulting in less stress on your feet and knees, as compared to running on asphalt and concrete surfaces. The extra cushioning and smooth surface also allows for easier runs.
You can simulate race courses.
If you live in a very flat area and you're training for a hilly marathon, you can still run hills on the treadmill.
You can simulate race conditions.
If you're training for a warm weather race but you live in a cold climate, you can run on a treadmill.
It's safer than running outside.
Running indoors on a treadmill means you don't have to deal with potential hazarads for running outside such as cars, cyclists, dogsand other attackers. If you don't have a running partner or groupto run outside with, running on a treadmill is a safer alternative than running alone outside.
You can control your pace.
When running outside, some runners have a hard time maintaining a consistent pace and end up running faster or slower than they wanted to. The treadmill makes it easy to track your mileage and pace.
You can multi-task.
Unlike running outdoors, it's safe to use watch television, or flip through a magazine while you're running on the treadmill.
Treadmills offer more convenience than running outside. You can run inside when it's late at night or pouring rain. Moms and dads can run at home without having to pay a babysitter.