Ok got it. When I did my OBA setup I learned 2 things: the first was the fewer airline connections the better- at first I had T's all over the place making my sliders into air tanks, multiple places to connect the hose to, etc. 6 months later I was chasing leaks like mad, my breaking point was I actually had a "blow out" on the way back to Reno, just after we aired up at the end of Rubicon- sounded like someone shot out my tire.. but was just a red air line popping off that had worked itself loose. I was happier when I cut all that out and just ran an air hose from the manifold back to the tank, bypassed the sliders, etc.
The tank is the second thing. In addition to more meat behind airing up tires and running tools, you can shut the truck off, and still have air available. So later on at camp you can top off a bike tire, fill a football, blow dust out of a radio, etc. You're not going to fill a tire from it, but having a little air around can be handy.