There are a couple of times when it makes sense to repitch yeast. One is if there is a stuck fermentation. Each case might be different, so it really would depend on the original OG, the current SG, the projected FG, etc. Sometimes the best course of action is to make a different beer, and then use the resulting yeast cake for the starter for the stuck fermentation.
The other might be if you have a beer that has been lagering for a long time, and you want to bottle condition it, or a big beer that needs a bit more yeast to carb up. I usually just mix up the priming solution, let it cool, and then add 1/4 package of dry yeast (nottingham is good for this) to the priming solution, stirring well. Then I rack the beer into it and bottle as normal.