Why should I bother with writing exercises when I’m busy with my manuscript, you ask.
Exercises don’t just serve to learn a skill but also to maintain and improve it.
Let me draw an analogy with sports and take an example from swimming:

I go to swim practice 2-3 times a week on average and have done so for many years. When I do my main strokes (front crawl and back stroke), I no longer have to think about what I’m doing, not even during flip turns. (That’s when you reach the end of the pool and turn around as quickly as possible and push off the wall.) So it’s possible I get lazy and make sloppy moves. But when coach gives me drills to do, such as touching my shoulder with my hand at each stroke or close my fists, or breathe at changing numbers of strokes (3-5-7, for example), I have to concentrate on what I’m doing and focus on my strokes (arm strokes in these examples).

It is the same with writing—there are “moves” that you don’t think about until you’re forced to think about them by doing a writing exercise. As a result, you are more conscious about these particular “moves”.