Sketchbook warm-up exercises are an essential part of art practice whether you're a beginner or a pro. Just like when you hit the gym (not something I do very often if I'm honest), it's better to ease into sketching rather than jumping straight into your ful flow.
With sketching, you're not likely to suffer an injury if you don't warm up, but sketchbook warm up exercises do prepare you physically and mentally for the task ahead. They help you relax your fingers, wrist, and arm, which can help improve line quality, coordination and also confidence in your sketching, and they can improve concentration, easing you into an observational and creative 'zone'.
