Well, I never heard of OCT until this minute, but a fiber rotator is generally for selecting the axis of polarization entering (exiting) a polarization maintaining (PM) fiber. A couple minutes with Google and I see polarization sensitive OCT is a thing, so I think that is the idea. If the light source is polarized, the scattering tissue can change the polarization. Different tissues give different polarization effects, so being able to change what polarization you are observing provides extra tissue differentiation and contrast.