At a recent job the client that I was working for wondered out loud why the walls in the house looked so bad--the paint came off very easily. I asked her if she had sanded before she painted. She said no.
The purpose of sanding is to create a surface for the new coat of paint to grab on to. The only time that it really is not necessary is when painting over flat paint, but even then I sand just for extra piece of mind--except on flat ceilings of course.
At another job, i was working on a stairwell and the paint on the skirts was peeling off very easily.
There really is only one solution to this problem: scrape, wash, sand, clean of dust and paint. I could have just sanded and painted. However, the substrate, the old paint that I would have been painting over, was still unstable--it was not adhering properly. And the new paint would not look good long. One ding, which is easy on stairs, and the peeling would start up again.
Luckily there was only a little bit of scraping to do. Imagine if I had en entire wall, or many walls that this was happening. An, easy repaint will quickly become very expensive.