Prior to 7854bd7109, this was used by the debugger for the OpenGL and D3D9 plugins to control logging (via PRIM_LOG and INFO_LOG/DEBUG_LOG in VideoCommon code; PRIM_LOG was changed in 77215fd27c), and also framedumping (removed in 64927a2f81 and 2d8515c0cf), shader dumping (removed in 2d8515c0cf and this commit), and texture dumping (removed in 54aeec7a8f). Apart from shader dumping, all of these features have modern alternatives, and shader source code can be seen in RenderDoc if "Enable API Validation Layers" is checked (which also enables source attachment), so there's no point in keeping this around.
Scott Mansell