Most of time walls don’t really need sanding. Generally look at the walls and feel them. If they look good and feel good and are clean they wouldn’t need sanding.
The times when walls do need sanding is when the roller from the previous paint job had left stipple or roller hairs stuck to the wall, or the paint job was so bad that it left roller marks such as what is on a orange. These would definitely need some sanding to remove the bumps. Another time you will want to sand the walls is when the walls are shiny and you want to apply a flat finish latex on top of it. Sanding the walls will ensure adhesion. If you are trying to paint latex on a unknown shiny surface you might be painting over top of a gloss oil or semigloss oil paint. And as you would imagine the latex doesn’t stick very well to glossy oil paints.
And sometimes as you will see in the video below for example you might have had a mirror or something on the wall that left some serious drywall holes and damages that you will have to do a skim coat of plaster to hide the imperfections and holes.
