I cannot believe that there are apparently people around who don’t think that practicing scales matters. Are you all nuts?
1) Scales teach you keyboard geometry, so that you know the “shape” of a given key in your mind. This will enable you to play a piece in a given key with a whole lot more fluency; when you see a dot on the lowest line in the upper staff and your finger reaches for what is in effect the F, it will be the scales that have made that happen automatically. When you play something in a given key, say AM, your finger will reach up automatically when you see a C on the staff, anticipating the sharp. Enough experience with the AM scale, and the Cnat won’t be part of the landscape if you’re playing in that key, not without a nat in front to catch your eye. This will enable you to catch all the accidentals with zero effort.
2) Scales teach you proper handshape and how to position your hands to maximize reach and nimbleness. You want 2-3 centered over the two grouped black keys when your hand is hovering around that area of the keyboard. And you want 2-3-4 centered over the group of three black keys when your hand is in that region. This will enable you to move fluidly and play pieces with no fingering written on them. Until I looked and said to myself, “Hey, there’s no fingering on this,” I didn’t even realize that fingering wasn’t written on the Ginastera. I didn’t need it. Why? Scales.
3) Scales also teach you expressiveness independently of other musical attributes. You can play a scale like a waltz, march, or rag. You can play a scale like a tripping little cute fluffy springtime song, like a patriotic song, or like a funeral dirge. Same notes. Big difference. And you can practice that difference and learn to hear it and create it more when the notes are just carriers for it. A really good actor can turn a count from 1 to 10 into either a comedy or tragedy at will, just by varying expression. So can a musician. So should a musician.
People are insane. Play scales. Just play the stupid things. Students who don’t play scales will never know why they should, and teachers who don’t teach scales probably wonder why some of their students can play an entire piece without even noticing that they are missing every single sharp and flat. “What’s wrong with these kids?” they ask. “It must be all that pop music they listen to!”