Most wines are still, that is not sparkling. Once a wine is finished fermenting, there is still some co2 in it but since it's not fermenting, it's not forceful enough to push co2 out the airlock. So, some co2 is trapped inside the carboy. Usually time takes care of it, especially if the wine has undergone several rackings.
Sometimes, though, the wine still has some co2 in it. This is common in kit wines, which rush you to bottle. Or in cooler fermentations, the wine "holds" onto the co2 more than in a warmer area.
Because a fizzy wine that is supposed to be still is unpleasant, the wine is degassed if it's fizzy. That's the only reason for degassing, and it's only done if necessary. It's usually not necessary.
Degassing doesn't have anything to do with bottle bombs- bottling too early is the cause of bottle bombs.