For traditional braces they put in metal spacers to create room between the teeth and by doing that, no enamel is removed from the teeth.
The spacers were to make room for the bands to be slipped over your molar teeth. Sanding (interproximal reduction) is used so that the upper and lower teeth can fit together properly when just a little space is needed to accomplish that -- it is an orthodontic thing, not an invisalign thing. It is also used to reshape teeth so when straightened there is not a gap at the gumline like a black triangle where the gums don't fill up that space even though the teeth are touching their neighbors, especially in someone who's gums may have receded a bit.
