Boiling before breaks down the fat and renders it out making for a mealy tasking brat afterwards. Skip the pre grill boil. Cook them long and slow so that they are evenly very dark brown on the outside, turn with tongs, not a fork.
Soak/simmer after. Make a bratwurst hot tub in one of those 13x9 throwaway foil pans, Slice up an onion, but a 1/4 to 1/2 stick butter, some coarse ground mustard and fresh cracked balck pepper and a can or two of American Lager (Bud, HighLife, Hamms, etc) enough to almost cove the brats. Let them simmer on the grill in the hot tub for anywhere from 30min to 3hours. Makes the party easier when you cook before your guests arrive and the food is ready whenever, buffet/sterno style...