Reading this project emphasizes to me that, if you're a maker and you aren't yet a master of power supplies and wires and motors and whatnot, you can short circuit (well, okay, hopefully not short circuit!) the process very effectively by just learning how to do this one project. Thenceforward you can confidently know you have the building block "computer fan motor" in your capability toolbox. You can then make many types of things move just by plugging this particular prime mover setup into your design.
Eventually, of course, it would be good to learn how to use the thing in greater depth. Perhaps you've followed that author's advice and ignored the third wire. That's cool. But some day in the future you'll be happy to learn what additional functionality that can provide. I believe it's a tachometer? Or maybe a thermostat on some fans? Obviously that day is still in my future, but I'm looking forward to it!