One is surge capacity for occasional needs. While the federal government is paying for some road building somewhere all the time, they might not want to hire permanent civil servants in a particular place for a project that's only going to last a few years. There's some sense to that.
Another category is where the government is buying goods or services off the shelf that lots of other entities buy too. That's your fuel example. Here again, there's sense to not duplicating effort.
But the last category is where the government is the only buyer and it's a permanent need. And that's where I don't see any sense to it.