Just like how we used to be able to select "compact."
I understand that more and more people are using vertically oriented devices like phones and tablets, but there is still a huge market share of users on desktop devices, and with the market saturated by widescreen displays, taking up vertical space while having 4 inches of white space on the left and right side of practically every website makes no sense.
I switch between Firefox and Opera because I really enjoy the sidebar that Opera has as it makes much more efficient use of screen real estate.