This is an interesting question. I probably can write a book about it with my answers
I have been practicing the last 8 years and I developed some crazy training methods and
I tried around a lot.
I try to answer the why:
Because it helps me to draw better and faster and it looks better and it´s also important to train
yourself to see what´s off in your drawings
Here are some things I practice:
line control, drawing straight lines, CSI curves, line weight
gesture, anatomy, figure drawing
perspective
faces, facial features
3d shapes