That's just to give some white space on the left, a stylistic issue. The second column is 55 ex. That is, it should get about 55 average characters on a line. You may want to adjust that to your taste. With normal font sizes, we want to limit the line length.
Text is unreadable if the lines are too long.
But we want the limit to be the number of characters on the line, not a width in pixels. That's because different monitors may need different font sizes, and people with limited sight may need very big fonts. Unfortunately HTML only allows us to specify size in pixels or percent. To specify in number of characters, we have to use a style sheet. Browsers without style sheets will use the whole width of the window.