Some benefits of running for a certain duration instead of a calculated mileage:
1. If you're not focused on pace, you'll run based on the appropriate effort level for that day. So, on days when you're not feeling great or the conditions are dreadful, you won't force yourself to go for your miles or feel guilty if you don't cover the distance. Instead, you'll run for your goal time at the right pace.
2. It helps you prevent boredom and explore new routes more easily because you're not worried about the mileage. Running a set route to cover your miles can be too rigid.
3. If you always run by distance, you might be tempted to push the pace and always hit a certain pace per mile during each run, rather than varying the intensity of your workouts.
4. It makes running unfamiliar routes simpler. If you are away from home you can just chuck on your running kit and head out for a set time.
Don't throw away that Garmin just yet! It's always good to do a regular tempo (race pace) run to check your pace and see how well you're doing.