I can't tell you why other than it is how the water plays off of the rudders (for walking). As far as why they give a good "back", I would assume they are pulling "clean" water when astern. I'm NOT saying that outboard turning wheels won't back or walk, what I'm saying is that inboard turning wheel boats are better in those specific ops. Watch a tug operator in any busy port and observe dockings and undockings. You will see boatmen walking not just light tugs but barges as well. To "walk" the barge, you need momentum (swing) on the tow, and the rudder gets shifted away from the intended direction, with reverse on the inboard side and ahead on the outboard wheel. Before everyone gets their thong in a wad, NOT every barge (or boat) can be made to walk.
Two VERY experienced captains that I know were attending a company required Ship handling simulator course at a VERY well known school. They asked the instructor if the Navy ship (that was on the simulator) could be made to walk. The instructor assured them rather smuggly that it could not be made to walk because it was exactly like the handling characteristics of the ship. Well , they made it walk much to the disbelief of the instructor....he didn't have much to say after that.
Now that all that has been said, should you "walk your boat" just because you can? NO. It is just another tool in the box. It is the most over used manuver when a little bit of rudder would have been just as easy.
Just like "palm Beaching it" (another show off manuever) , walking is sometimes used to impress audiences at Shooters rather than a necessary docking tool.