When the weather and footing are bad
This is the most obvious benefit of treadmill running, but it’s important to include because elements effect every runner differently. Personally, I have a very difficult time when it’s hot or there is bad footing; however, put me on a clear road on a cold or rainy day and I’m a machine. You may be the opposite, so don’t be afraid to hit the treadmill on the days you need to.
Getting in a good workout on the treadmill is better than suffering through a bad run or getting hurt.