It's easier on your body.
You don't have to worry about the weather.Indoor running means that you don't have to run in bad weather, such as heavy rain or snow. You also don't have to run outside when it's extremely hot and humid. Running in the comfort of a temperature-controlled environment also means that you don't have to worry about overdressing or not wearing enough for your runs. In addition, you don't have to worry about putting on sunscreen before your run.
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.
More: Treadmill Hill Workout
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 to get acclimatized to a warm environment.
It's safer than running outside.Running indoors on a treadmill means you don't have to deal with potential hazards of running outside, such as falls, cars, cyclists, dogs, and other attackers. If you don't have a running partner or group to 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.
More: 4 Boredom-Busting Treadmill Workouts
You can multi-task.Unlike running outdoors, it's safe to use headphones, watch television, or flip through a magazine while you're running on the treadmill.
More: Beat Boredom on the Treadmill