The obvious reason that people do this is because of wider and wider screens. Some web pages or applications still show text that fills the width of windows. If you have to turn your head back and forth, then you are much more likely to lose your line on a large block of text. It's much easier to pick out the next line with a line marker.
I'm always surprised people actually use the web browser in full screen. I usually divide my (23" 16:9 2048x1152) screen in at least two, sometimes three "panes".
This is a lot easier to do when using a tiling window manager -- but I hardly ever use the entire screen for the browser.
Still, even with just half the screen -- the browser pane is roughly equivalent to an A4 page -- way too wide for a single column of text -- so on poorly designed pages I frequently end up resizing the window to get a usable line width.
I should add that due to RSI I use the mouse as little as possible, preferably using a tiling window manager like awsome or xmonad, and vimperator for Firefox.
