Slow practice is used by the top musicians to make sure they really know their piece. Rachmaninov was notorious for doing this (and who can criticize HIM!?). But WHY practice so slowly?
when learning a new piece this can help to give your brain time to learn the piece
when you have a section that moves super fast - it can help you to gain control and develop better fine motor skills needed
what about places that need dynamics or ornaments? This is a great opportunity to get better at those details in your piece.
You already know your piece backwards and forwards? Try it super super slow and see what details you can add!
Memorization happens easier if you do it slow. (just sayin')
Slow practice is better practice according to the great musicians and who are we to argue with Rachmaninov?!
Thanks Practicing The Piano!