When terrorists took over the four US airliners on 11 September, 2001, one of the first things they did was turn off the transponders so the planes would not register properly on civilian radar.
If these had not gone silent, air traffic controllers would have seen two of the planes heading for Manhattan; instead they lost precious time trying to figure out where the aircraft were.
At the time it was generally assumed that transponders would be re-engineered to prevent them being turned off illegally but nothing was done and 13 years later along came MH370.
Switches were first installed to avoid grounded planes sending signals that disrupted airport radar, but modern ground-scanning radars do not get confused by transponders on taxiways.
In fact, private planes had dealt with the old radar problem by using automated transponders that turn on when the planes become airborne and turn off when they slow to taxi speed.
(Dr) John Cameron