Why does Unity use duplicate vertices in Mesh?

I'm working on importing .fbx files through a c# script. My current hangup is because of how Unity deals with normals and UVs. That is, the vertices, normals, and uvs arrays all need to be the same size, which feels pretty wasteful and arbitrary.

For example, a cube has 8 verts. However, Unity reads it as 24 verts, with I believe 3 representations of each vert (so, say, 3 v(0,0,1) verts). Looking at the triagles array, Unity also uses all 24 verts.

Does anyone have any idea of how to interpret the extra vertices? How do I know which vert is where, working in code? Why would the program use vertex[12] for a triangle instead of vertex[4]?