Material IDs are used to split a model's surface into multiple Materials.
Normally a model can only use a single material. However a number can be assigned to sets of Polygons which each correspond to a particular material.
When the surface is split to use different materials, this does not create a visible split on the model. However a game engine must duplicate the vertices along the seam, which increases the vertex count of the model in-game. These are called "material splits" (see Whitepapers).