The main difference is production quality. When these huge professional level companies master a track, they then go and test it on up to a hundred or more different audio devices (car stereos, home stereos, home theatre systems, walkmans, clubs, PAs etc etc) then they continue to tweak it over and over until everything is perfect. They need as much digital information as they can to do all of this. Another reason they do it is "because they can." :p

For a demo track or garage band recording.. you won't ever really need more than 24-bit 48KHz.. and 16-bit 44.1KHz is the most information you are going to be able to put on a CD anyway. I always record at a slightly higher resolution than I am going to output to so that there is no down-mixing messiness.