This is a long-standing question, and one that's been around since treadmills were extremely rudimentary machines. The short answer is, yes. For nearly all purposes, they can meet or exceed the ability of the great outdoors to help you on your fitness journey. Of course, none will precisely match the bounce of the soil on a good trail, or the atmosphere provided by ambient nature sounds. However, they will do things like reduce strain on your legs from running on less-than ideal-surfaces or provide you extremely precise feedback on pacing, speed, and heart rate. Most importantly, they will always be available, regardless of weather, available transport, or childcare. It doesn't matter how superior an outdoor run is if it never happens, and there are many, many obstacles that can easily turn a planned 5K into a Netflix binge instead.