Currently, the only way to disable default animations is to set an avatar model as 'Generic' which breaks VR and Full Body Tracking. This makes using things like a VR Treadmill impractical to use in VRC because you're either always in a walk animation or the animation isn't properly synced with your actual body movement.

By giving us an option to disable this, we would still be able to use FBT with Treadmills without issues, and would allow others to experiment and test new/developmental technologies inside VRC. It would also allow regular users using FBT to not 'jump up' into a moving animation if they're on the floor or sitting or whatever else, and allow a much more immersive experience for users that use this feature.

Edit; Grammar