This is especially targeted at buildings like towers and castles. The idea, is basically, if you want to heal units in a building, it is really difficult to tell which ones are on full health, and which ones are simply close to full health. There should be a new button on these buildings (Castles, Towers, Town Centers), something along the lines of “Ungarrison Fully Healed Units”. That way, if you are playing a campaign, or even a real game, and want to heal your units, it becomes a lot easier to do so with buildings, or at least checking the full HP units becomes easier.
Do people heal units that often using buildings?
Not usually. I sometimes do, particularly in a campaign. Only valuable units for a ranked game, when I know I have time, and typically a Castle as well. In campaigns, especially with a few units and certain types of scenarios, I will often heal infantry or archers in towers or other buildings. So sometimes, would be the best answer. Or, as T90 would say… It depends.