I second
@Don_ih's response.
Converting a color image to black & white requires a myriad of decisions. Do you want the folks who designed your camera making those decisions? Or do you want to make those decisions yourself?
One has an incredible amount of control when converting a color image to black and white in most software. One can adjust the tonality of each color in the final image. This process is sort of the equivalent of stacking filters one atop the other on a film camera with black and white film. Something that is/was not actually done because of the optical degradation caused by all of those surfaces.
I'd much rather make these adjustments myself while watching the effect on the image in order to obtain the creative effect I want than to allow the committee who designed the camera and its software make 'average' changes.
Furthermore, one can get really sophisticated with software and apply a different set of tonal conversions by making and masking layers. This is not something one does regularly, but it can provide wonderful creative control when needed to achieve your creative vision.