Two reasons! First of all, searing the pork gives you that golden crust which equals flavor and seals in the juices. As an added bonus, those little bits at the bottom of the pan after searing add major flavor to pan sauces if you choose to make one after while the pork rests. Second of all, the combination of searing and roasting results in perfectly cooked pork tenderloin. Only cooking on the stovetop would create a thicker band of overcooked pork on the outside before the center is cooked though and only roasting in the oven would result in missing out on the flavor from that golden crust and sealed in juiciness you get by pan searing first.