For one, it gives me a break from tackling my wrong notes, distasteful vibrato, and being torn between interpretations in the practice room. There are no wrong notes, no wrong inflections. I wouldn’t say that a note during improv with “distasteful” vibrato/intonation/whatever was necessarily done on purpose, but was made in the moment and without expectation. There is something very freeing and empowering about this. What happens on accident- a cracked note, or a gasping breath, can turn into inspiration for what is to come.
At the same time, I can tackle my classical music troubles through improv. Lately, I’ve had issues with controlling the style of my double tonguing. I’ll start moving my fingers, with no regard to scales or my piece, and focus solely on my double tonguing. This allows my mind to be entirely focused on the production of my tonguing, because I am not going to be distracted by the fingers in an awkward passage, or by the monotony of scales.