The sausages poach in the finished, hot spaghetti sauce for 20 minutes. During this time they absorb lots of delicious tomato-y flavor, and the sauce absorbs delicious sausage-y flavor. They also get a jump start on cooking, so the pan-frying is mostly just to get nice browning and crispiness on the outside. If you're in a rush, or if you're cooking for some vegetarian or vegan guests, you can jump straight to pan-frying them, but if you can poach first, it's worth it.