The idea behind soaking bratwurst in hot beer prior to grilling is that they cook all the way through ahead of time, so the grill master needn't worry about serving an undercooked sausage. The beer also makes the sausage casings more pliable and less likely to split.
At the end of boiling, let the brats rest for about 5 minutes before grilling, and after the grilling, they will be full of flavor, crispy, and very juicy.