None of this has anything to do with why you would want to spend money now vs. spend money later. You're not getting the DLC until September either way. Other than the issue of earning interest (which is negligible for a $40 purchase), the only difference between spending now and spending later is that if you spent the money now based on an assumption that having the DLC in September will be worth $40 to you, it's possible that when September comes around circumstances have changed and the DLC is no longer what you'd want to spend your $40 on, so now you have to try and cancel it, or perhaps it's too late and you're stuck with it, whereas if you wait to spend the $40, you can make the decision as to what to do with the money in September, when you will have much more information about how much you want the DLC vs. other priorities that the money could be spent on and the choice to use the money in a way that's most beneficial to you.

Preordering is basically an emotionally driven decision to spend money that's designed to prevent yourself from making a rational decision not to spend money.