According to this document under section 2.6 IFD #2 it's the uncompressed small version of the picture which has no white balance correction applied.
CR2 files are based on the TIFF file format so it would make sense for the uncompressed data to be in the TIFF format.
Its purpose is mostly for displaying the RAW histogram in tools like Digital Photo Professional based on the uncompressed image data.
The TIFF preview also appears to be rendered linearly, with no gamma correction applied, and the white and black points set at the extremes of the sensor's dynamic range.
