Why buy a PSU when your case already is supplied with one?

It’s been generally an issue, most specially with those building budget-PCs, is the PSU. Often, this component of the computer gets overlooked. Yes, you buy a neat motherboard, the best ‘bang for the buck’ AMD processor (sorry Intel, I like AMDs) and RAM, and yet, you settle for your PC case’s bundled Power Supply Unit (PSU). Now, lemme see… If I were an ordinary user, I don’t think I’ll ever use up all the 500Watts that the PSU ‘can handle’. But if you’re a gamer, an overclocker, or a performance user, then it would be easy to gobble up all the 500 Watts (which is actually just about 300 Watts if you really test your PSU). But is is just that?