you are on a road course going through a corner at about 70mph... you are at or near the traction limits of your car.... now if you were to just do a conventional downshift the car would jar and you will lose traction most likely going into a spin and possibly fucking up something BIG TIME... so you match revs to make the downshift smooth.... the car holds traction and you are now at your powerband to pull out of the corner with some authority