I think the origin of boiling brats (or any sausage for that matter) goes back to the notion of getting the pork in them fully cooked. We normally cook our stuffed breakfast sausage like that. Put them in a covered skillet with enough water to cover the bottom and let them simmer on low until the water evaporates (about 10 to 15 minutes), then remove the cover and the sausage then browns in it's own fat. In a covered pot, this is essentially cooking with steam, which gets all the way into the middle. Not boiling, just a slow simmer. A brat would work the same way. Simmer in water or beer until the liquid is gone, then grill it (and you can do this in a shallow pan on the grill).
But I normally just grill my fresh brats on low heat. That takes about 20 to 30 minutes, depending on temps. Hot enough to grill, but not so hot I get a grease fire going. If they are good and brown on the outside and no pink on the inside, they are done enough for me.