Beyond the aesthetic issues, a repaint can also hide repairs, or be necessitated by said repairs. Let's say you buy a bike and in overhauling it you notice overspray of a different color on the fork steerer. Is this a replacement fork, you wonder? Dents can be rolled out, have bondo added and then get a repaint. Maybe even a tube replaced. If it has the original paint, a buyer has a better sense of how hard a life the frame has lived.