Fresh paint has a way of making wall problems more visible, not less. A hairline crack that disappeared into the old flat finish will show up sharp under a new eggshell coat. A nail pop that was hidden by a picture is suddenly a shadow at eye level. Water stains bleed straight through standard paint within weeks.
That's why we treat drywall repair as part of every interior paint job, not a separate trade. Before any color goes on, we cut out and back damaged sections, re-tape seams that have bubbled or lifted, set nail pops, fill cracks with the right compound, and sand it all flat.
Stains are their own problem. Water, smoke, and marker need a stain-blocking primer, or they'll come back through no matter how many coats you apply. Repairs also need to be primed, because bare joint compound absorbs paint differently than the surrounding wall and will flash through the finish as a dull patch.
The advantage of one crew doing both is simple: there's no gap between the drywall finisher leaving and the painter arriving, and nobody to blame but us if a seam shows. It's also cheaper, because we're already there with the drop cloths down.