Why is web-page in portrait format?

Hi, I'm enjoying the Catch Everest theme, its clean and attractive. Just a question though. Why is the page arranged in a vertical (portrait) format when all PC screens are horizontal (landscape) format? It means viewers have to scroll down to see all the content and there is an unused portion of the browser at the sides (where the background can go). Is this to suit mobile phone browsing? Can I change the format? Thanks, Wul