Why the standalone shows with a voltage regulator?

Hello. I'm noob in this subject, and I was wondering why the arduino stand alone version needs a voltage regulator whether I could be feeding the circuitry straight with 5V? Thank you very much.

If it's a clean regulated 5v supply you could. Otherwise you could have stabilty issues or even damage things (5v wallwarts can output a lot morte then 5v)

Where do you get your 5V? A magical outlet or a 5V voltage regulator?