Reforming or converting makes it much easier to hold something together, while being unreformed makes it a lot easier to grow. Oftentimes, a good strategy is to grow to a huge size with one long lived ruler, and then convert to an established faith and switch your succession law. You get to keep everything, and it should be fairly easy to demand conversions from most of your vassals using such an old ruler. Reforming, rather then converting, has the advantage of keeping raiding abilities as well as a few other religions features (if you had them), and makes it easier to convert your lands.
