My primary motivation to train indoors is to keep my fitness up in the winter months, or generally, if the weather is bad, it still means I get a good workout.
Benefits of indoor training for me at a spin class are, a brilliant cardio workout for either 45 mins or 1 hour (depending on instructor), with no rest is a great way to push yourself, more so than maybe if you were out on your bike. All classes are done to music, using the rhythm of the track that’s playing to your pedal stroke. This gives an effective way to pace yourself when tackling climbs, standing or seated, which I have found of great benefit.
My advice for those wanting to keep up their fitness when the weather doesn’t allow them to cycle outside, is that a spin class will give you a great cardio workout and teach you techniques that are useful for outdoor riding too.