Why do you need to separate the different polarizations of light?
Any light wave can be described in terms of horizontally and/or vertically polarized light. For most things, these two polarizations are unimportant. But when light reflects from surfaces or passes through certain materials, these polarizations become important. The charges in surfaces and materials do not always respond equally to the two polarizations of light. The two polarizations may even travel through very different paths (e.g. in the polarization beam splitter).