For each rank up, you can order another weapon. Rank Two lets you order two. It also lets you pick variants of the gun, so Rank Two would also let you choose between two different variants of the same gun. So on and so forth. Also, hitting Two and Three unlock a Class-Specific Legendary and Exotic Quest, respectively. On top of all of this, you get a random weapon from attaining a higher rank. I just got a baller Omolon Scout Rifle for free the other day.