Touch-up paint only looks good on a smooth surface. If there are small bumps, roller lines, old drips or rough patches, simply painting over them keeps the defect and sometimes makes it more noticeable, especially in side light.
Lightly sanding with fine sandpaper smooths these high spots, feathering the edges into the surrounding area. Then when you apply paint, it sits evenly and blends better. The aim isn’t to strip the wall, just to knock down the obvious imperfections.
A few extra minutes of preparation can be the difference between “Wow, that mark is gone” and “Now I see a shiny blob exactly where the problem was.”