The bigger it is, the wider that is useful. Basically you find that you need a certain amount of vertical real estate to work effectively. So on a small screen like a laptop, a 4:3, or even more square, monitor can be of use. However when you start getting large desktop displays, wide is very nice. Personally I like 16:10 displays for the desktop, in part because I find them aesthetically pleasing (likely because they are near the golden ratio) but also because for the large sizes I like (30" currently) it provides a good amount of vertical real estate, but plenty of horizontal to fill my field of view and allow for multiple things to be displayed at once.
For TV, heck I could go even more than 16:9 if such a thing were standard. I was always partial to 1.85:1 3 perf and 2:1 Superscope for movies myself.