Just to clarify: the landing gear is by default linked to the common hydaulic system, so if the main sistem fails, your landing gear will operate as always. However, if common hydraulic system fails, then you have to use the switch we are talking about, to make the landing gear operate from main system.
BTW, once landed, ground crew do not repair the hydraulic if you do not have other damage, it's curious. On another note the hydraulic accumulator only loads presure from common system.