It is about improving the packing of grounds by breaking up the subtle clumping that occurs during grinding to reduce channeling where water finds an easy path so part of the puck is over extracted and other parts are under extracted.

If you already had perfectly even flow, it's not going to improve anything. That might be true with great equipment or a very forgiving bean. It might also be true if WDT isn't enough to fix your channelling alone so there is more left to fix. This leads you to watching slow-mos of your pulls and inspecting pucks :)