I never took a graphics class before, can someone explain, or direct me toward a resource that explains why triangles are used as the primitives for rasterisation? I assume its because they are the simplest 3 dimensional shape and you can break up any image into groups of 3 points to cover the entire image
A really nice property of 3 points in 3d is that they will always lie on a plane. If I gave you 4 non-coplanar points in 3d, those points wouldn't define a specific shape the same way 3 points always do.
Here's a stackoverflow post on the topic if you want some more information.
I believe that renderers are designed to handle them efficiently since triangles are a 2D shape with the smallest number of vertexes which makes them easier to compute on. Other primitives will then start to be included if they cannot be made easily from just triangles; such as circles.