AFAIK, the round butt helps conceal the gun better by reducing printing.
Round butt S&Ws can use either round butt grips, or conversion grips, which are designed to fit round butt guns, but have the exterior dimensions of a square butt grip. Check out the above pic of the Uncle Mike's for instance. I find I shoot conversions grips better than round.
Since they look like square butt grips, be sure to look for the word "conversion" in the description, since square butt grips obviously won't fit a round butt gun.