So what exactly is the point or need of using a pressure cooker to do it?
The water bath, by it's nature, can only heat the food being canned up to around 212 degrees (a.k.a. boiling point). Not all bacteria are killed at boiling, some of them are pretty tough little buggers. By using a pressure chamber, you can increase the heat beyond the boiling point and kill those bacteria as well. You can get away with water bath when canning higher acid foods and such because those more heat resistant bacteria can't take the higher acid, so they're not in the food to begin with.