Power supply, you can install it right when you get it and it’s usually one of the cheapest components (Case/HDDs aside).
If you know what you’re going to order, and are able to purchase them all at once of course, why not bulk buy parts, like your drives or peripherals?
(EDIT: Turns out, tab doesn’t 5x space, it posts:D )
Well if I sit too long on my pay check I tend to spend it, therefore I thought if I buy it piece by piece than it will all come together in the end anyway. Although I am buying somethings together like hard drives, and RAM.