Q&A: Why Should Beginners and Intermediate Musicians Play Scales and Other Technical Exercises? (Good Question)
tonymiceli
11 May 2015
Because you're not in an environment where you are playing and keeping your hands in shape.
If you can blow over Cherokee at 300 BPM then you probably don't have to play technical exercises if you don't want to. You can just blow over tunes and work out your hands.
But for a lot of players new to the instrument you can't do that, so then it's important to play scales and other technical exercises and to work up speed.
Not much more to say about that. IMHO :)