The benefit of obtaining your permit online is to skip the LONGEST part of the line - which is paying for the permit. I remember spending hours upon hours at the border crossing in Laredo every summer while my dad worked out getting the permit for our Econoline E150. Now we don't drive as much anymore, but some of our relatives still do and they obtain the permit ahead of time. They wait in a shorter line which consists of an official coming out to inspect the vehicle (making sure it matches the permit you pulled) and applying the permit to your windshield.