A typical camera sensor does not capture RGB pixels, but instead captures input distinct red-sensing pixels, green-sensing pixels, and blue-sensing pixels at slightly different locations; a raw file will report the values of the individual pixels as captured.
When a raw file is converted to an RGB pixel format, each pixel in the output file will typically be a weighted and filtered average of a number of pixels on the original sensor. Once the data is converted, each pixel in the resulting file will be capable of independently representing any color. If one wants to e.g. adjust the saturation in a file of RGB pixels, each individual pixel's red value can be based upon its blue and green values, and likewise adjust each pixel's blue based upon its red and green, and adjust its green based upon its red and blue.
If one wanted to apply a white-balance adjustment to a raw file, however, one wouldn't be able to adjust the color of individual pixels, since each individual pixel is only capable of sensing a single lightness value. If one wants to reduce the saturation of a raw image of a red object, it wouldn't be possible to increase the blue and green values of all the red-sensing pixels; instead, one would have to increase the reported values for blue-sensing and green-sensing pixels that were near brightly-lit red-sensing pixels. Such operations are not difficult, but each time they are applied will degrade the image a little more. By contrast, the act of converting sensor data to an RGB picture is generally lossy, but such loss need only be incurred once.