Soap solution wets and penetrates a surface easily. It spreads well and smoothens the surface. The surface becomes slippery.
The fingers, wrist and hand when they are dry, there is a lot of friction between the bangles and the parts of hand.
When soap solution is applied, the friction (coefficient of friction) is reduced greatly. Also hand becomes softer. So bangles slip on the surface of hands more easily, as compared to when hands are dry.
Hence, there is less pain, less heat generated when bangles are worn and also, there is less stress/force on the parts of the body.