Fresh paint makes every flaw underneath more visible, especially in rooms with strong side light from windows. Nail holes, dents, and patches that weren't sanded flat all show once the wall has an even sheen.
Good prep starts with filling every hole and crack, sanding it flush, and spot-priming patches so they don't flash through the topcoat. Gaps between trim and wall get caulked so the cut line looks crisp.
Then comes protection: floors covered, furniture moved and wrapped, outlets and hardware taped or removed. It takes time, but it's what keeps the job clean.
With the prep done, the painting goes quickly: cut in along ceilings and trim, roll in even coats, and allow proper drying time between them. Two coats of quality paint over a well-prepared wall will look better and last longer than three coats over a rushed one.