Most of the time when you add yeast at bottling, it's to ensure carbonation.
This is normally not a concern for average beers, but can be a concern with aged beers (barleywines, sour beers, etc.) that have been aged for 6 months to a year.
Other times, you can pitch a yeast at bottling time to develop different flavors. For instance, I brewed an Orval clone with the Wyeast Trappist Blend at the beginning of this year. I let it be in the primary for a month, the secondary for 3 months and dry hopped it for 2 weeks. When it was bottling time, I wanted it to be more "authentic" so I built up a starter of Orval dregs and pitched it in at bottling time. The beer has really changed a lot over time and has really picked up the tart/funky aromas and flavors that Orval has.
In normal circumstances, it is not necessary to add more yeast at bottling.