The bottom line, is Triangle Rasterization, which is how computers render objects to the screen. Though others say it more elquently than I:

All 3D objects that we see on the computer screen are actually made of tiny little geometrical objects often called primitives. Quadrilaterals, triangles, n-gons etc. are example of primitives. We will concentrate on triangles mostly because of one main reason: every object can be split into triangles but a triangle cannot be split into anything else than triangles. Because of this, drawing triangles is a lot simpler than drawing polygons of higher order; less things to deal with. This is why those triangles are so commonly used in computer graphics.

Emphasis mine. Source: http://www.devmaster.net/articles/software-rendering/part3.php

The bottom line is Triangle Rasterization, which is how computers render objects to the screen. Though others say it more elquently than I:

All 3D objects that we see on the computer screen are actually made of tiny little geometrical objects often called primitives. Quadrilaterals, triangles, n-gons etc. are example of primitives. We will concentrate on triangles mostly because of one main reason: every object can be split into triangles but a triangle cannot be split into anything else than triangles. Because of this, drawing triangles is a lot simpler than drawing polygons of higher order; less things to deal with. This is why those triangles are so commonly used in computer graphics.

Emphasis mine. Source: http://www.devmaster.net/articles/software-rendering/part3.php