The second reason one would want to color-sand and rub out a finish is because this process can be an absolute savior for any paintjob that didn't turn out as well as expected. There's an old adage amongst painters that if you don't get any runs you're not trying hard enough. That's hard enough as in trying to "slick it," a term meaning a painter did a perfect job of laying the paint on to look as slick as glass. Performing such a feat is every painter's ultimate goal, but there's going to be some beginner's dues to pay along the way. These dues will come in the form of runs, fish eyes, dirt in the paint, and most likely some paint flaws that no one has ever seen before. No matter what the flaw or blemish might be, color sanding, followed by a thorough polishing (rub out), can save the day.
The third motivation for color sanding and rubbing is to revitalize a paintjob that for any number of reasons has lost the appearance of new paint. It's only a matter of time for an automotive finish to lose its glossy sheen due to the effects of ultraviolet light, extreme heat or cold, or even the quality of the paint that was used. Typically a color-sanded and rubbed paintjob using lower-quality paints is more subject to a condition that's known as dull back. In some cases, it takes only a few months before the shines goes flat, and if left unchecked, the finish can deteriorate to the point of flaking and peeling. If caught in time, the ill effects of allowing a finish to deteriorate can be delayed or avoided all together.
There is an expression used by painters that's known as breaking the skin. There was a time when any paint that was color-sanded and rubbed wouldn't hold up as well as paint that was allowed to keep its skin. In the 21st century's second decade, the advent of superior-quality, high-solid paints means a color-sanded and rubbed finish can endure as long as an untouched finish. That's great news for DIY guys and professionals alike because show-quality results are now easier to obtain and will endure for a long time.