There are five reasons: 1) Every time you rank up, you get a legendary weapon 2) You can place X each week where X is your gunsmith rank 3) For each order placed, you will get to choose between X perk sets where X is your gunsmith rank 4) At rank 2, you get a quest for a class specific, legendary, secondary weapon 5) At rank 3, you get a quest for a class specific, exotic, primary weapon