Why do airplanes need oxygen masks when there is an accident?

This thing is only an emergency equipment, only appear in the cabin appeared to lose pressure situation, only by the computer automatic control fall down. If it is such as the landing gear can not be put down, the engine stops, the oxygen mask will not fall down. In addition, because this thing is supplied with oxygen by the oxygen cylinder, so can only last less than 10 minutes, this 10 minutes is to give the pilot to quickly reduce the altitude, if more than 10 minutes, the aircraft is still in the stratosphere at high altitude, that have this mask is still a dead word, history has this tragedy.