Power
Like Rihanna said many, many moons ago, "Shut Up and Drive." Literally. Couriers are on the road pretty much 24/7, so there's not really any opportunities to make any executive decisions around the office. A courier is told where to go, and makes sure the truck gets there. Simple as that.
You might not have that much executive power, but you're still a federal agent. With that comes a certain level of authority on the road, especially when transporting hazardous materials. Now, we're not saying you have the power to call in a police escort so you can avoid stoplights.
But if you ever find yourself in a situation that you deem dangerous to you and the materials sitting in your truck, you have the authority and the guns (literally) to defuse that situation. When people see a federal uniform in their presence, they tend to shape up.