Its pretty straight forward, if you own a plot with an estate on it, this move feature saves you from loosing your current plot, because prior to move you must demolish and relinquish (now anyone can buy your plot) this way the system does it for you as you stand at the new plot to purchase, saving you the prior steps plus moving your character to the new housing area, you'd most likely get beaten to it and then loose out and have to buy another plot you didn't want. As for the old plot purchase cost, i don't see why they would return your gil, this would open up a new can of worms, players buying and selling plots and making 100% profits, nobody would be getting a house then without paying someone else first...