The site splurges out to fill the full width of a modern widescreen display with text. This style pulls the whole body of the site in to a more reasonable width in the center of the browser window and then picks out individual elements such as paragraphs to narrow even further to around 14 words per line which is far more readable [1].

It does not affect any other aspect of styling, thus allowing you to mix it with independent user styles for color, font choice, etc.

1. http://tex.stackexchange.com/a/13877