Mountain biking is so much more enjoyable when you’re fit, and if you have more matches to burn getting up that techy climb, you will have that much more to give on the descent. The best way to get fitter is by doing structured interval sessions that are designed to push specific systems of your physiology; the trouble is that doing intervals and hitting specific power targets on singletrack is not really all that feasible. It can be downright dangerous not only for yourself, but also for those around you — plus, it’s a fantastic way to ruin an entertaining bit of trail.
Riding indoors removes all the variables and allows you to complete structured workouts more efficiently; if a workout calls for you to do 320w for 10 minutes, there are no hills, rollers, rock gardens, or downed trees in the way and the only thing stopping you from hitting your targets is your legs.