Human beings, like most other creatures, are equipped with two eyes, situated close together and side by side. This positioning means that each eye has a view of the same area from a slightly different angle. You can check this out by focusing on a distant object and viewing through each eye alternately -- see how some things seem to change position slightly?
The brain takes the information from each eye and unites them into one picture, interpreting the slight differences between each view as depth. This produces a three-dimensional picture: one with height, width and depth.
It is the added perception of depth that makes 3-D, or stereoscopic, vision so important. With stereoscopic vision, we see exactly where our surroundings are in relation to our own bodies, usually with considerable precision. We are particularly good at spotting objects that are moving toward or away from us, and the positioning of our eyes means we can see partially around solid objects without needing to move our heads. It's easy to see why some people believe stereoscopic vision evolved as a means of survival.
Certainly, stereoscopic vision is vital for seemingly simple actions such as throwing, catching or hitting a ball, driving or parking a car, or even just threading a needle. That's not to say such tasks can't be managed without 3-D vision, but a lack of depth perception can make these everyday tasks much more complex.