You don’t always need to sand every inch of every wall, but some level of surface prep is important. At minimum, you should lightly sand glossy or uneven areas, and smooth out any peeling or flaking paint. This helps the new paint grip properly and prevents old imperfections from showing through.
If the existing paint is matte, in good condition and you’re repainting in a similar colour, a quick, light sanding to knock down rough spots is usually enough. However, if there are multiple old layers, shiny patches, or previous roller marks, a bit more effort pays off in a smoother finish.
Walls with serious issues – like cracking, bubbling or heavy patchwork – may need more thorough sanding and even application of putty or filler in places. Skipping prep can save time today but often leaves you with visible flaws that annoy you every time light hits the wall at an angle.