It's not about worrying about enough fuel for the boiler, but not having enough boiler for the load. Imagine you had steam engines on all your machinery - to run them all at once you'd need a massive boiler, but you don't do that, and having such a massive boiler just-in-case results in a load of wasted fuel (more and more the longer this goes on). So you buffer your boiler output, and now you can run those machines for a few minutes without having a ridiculous boiler to do so.