I need to sand down the painted parts of my current project because I screwed up - I got some debris in the paint job. I think some of it is chunks of other colors of paint that I hadn't properly cleaned out of my airbrush, and some of it is probably pet hair or dust or whatever... It always seems to happen on my projects... very frustrating. Sanding can cause all kinds of additional problems, of course (like, in the worst case, scrape some of your paint off entirely) but it can be helpful for erasing minor errors sometimes.
I also usually like to sand down the clear gloss coat that I put over decals - if you give the model a heavy gloss coat after decaling, you can then sand down the clear coat and almost entirely erase the border between the decal and the surrounding areas of the part.
Otherwise, I don't think there's usually a reason to sand painted parts.
