Pixel depth is increased to provide the space to store 'nodata' values. This allows changes, such as a shift or a mask, to be applied to the raster. Pixels within the new raster's bounding extent may need to be assigned as nodata to preserve data values and also keep a proper extent.
For example, values range from 0-255 in an 8-bit raster. If a nodata value is introduced, the bit depth must increased to 16-bit to store the nodata value, which would be the 257th value.