What does this accomplish?
Keeps 'em rounder.
And they will lose air while they sit. If you overfill them, they might not be flat when you pull the car out of storage.
It accomplishes (or tries to) not having flat tires when you get the car out of storage.
Yup, flat spots.
When the weakest points in keeping air in the tire are the bead to rim seal and the schrader valve seal and both increase in their sealing effectiveness as pressure increases wouldn’t it be logical to inflate the tire to its maximum safe limit?