Screen-Space Reflection
Probably the most viable reflection technique available for #2385, #6221 and #6213, which are highly dependant on reflections to look as they are intended to.
Multiple water levels (#2385): OpenMW's planar reflection becomes unviable when there is more than one plane of water to be rendered.
Non-RTT path for water shader (#6221): RTT reflection and refraction is an extremely expensive operation since you're drawing virtually everything on screen a second time - double the draw calls, double the triangles.
Support for PBR Materials (#6213): A huge part of PBR's strengths over traditional rendering is how it renders reflective materials. But in order to look reflective, it needs to be something to reflect. For real-time graphics, SSR is one of the main techniques used to accomplish this.