How much do you know about HTML layout? How much do you know about Cascading Style Sheets? How much do you know about the Dao of Web Design?
Once you understand these core concepts of the web, then you'll understand that there's many ways to skin the cat and that the web is...flexible. Yes you can make your content spread across the entire browser screen, no margin or padding from the edges. But it doesn't make for a good reading experience. So then you start adding padding and margins to make things look nice. And pretty soon you end up at something like twenty-twelve. All the content is in a nice central area, with a good width, and some nice padding and margins to make it appear centered in the browser window. Or twenty-fourteen which decides that putting all the content into a left-aligned column works best and the stuff to the right can be decorated with background images if so desired.
So to sum up - you can do pretty much anything - left aligned, justified, centered, no margins or padding, lots of padding and margins, no set widths, very specific widths, or more flexible percentage-based widths, or whatever.
But just because you can do something doesn't mean you should.