The Streaming Controller component is part of the Mipmap Streaming system. Add a Streaming Controller to GameObjects if you want to use Texture Streaming to load their mip maps into Unity at run time.
For more details on how to use the Streaming Controller component, see the documentation page Mipmap Streaming API
|Mip Map Bias||Use this setting to force Unity to load smaller or larger mipmap levels than the one that the Mipmap Streaming system has chosen for those Textures. Use the numerical field to set the offset that Unity applies to the mipmap level. Unity adds this offset to all Textures visible from this Camera.
To control this via API, use StreamingController.streamingMipmapBias.