To prepare any surface for painting, you need to remove all loose material like cracked or crumbling paint, and any surface contaminants like grease. Scratching the surface with an abrasive provides a 'key' for the paint to adhere to. So, even if the old paint finish looks good, running a power sander over the wall is good practice. The best kind to use is a quarter sheet, palm grip sander like the Makita mentioned in the article. Palm grip sanders are powerful, comfortable to use in one hand and not too heavy to use for long periods. You can even use one on a ceiling if you are reasonably fit and strong. Always use a face mask with these machines, they can produce a lot of dust!