Because you don't know what the final design spec might need to be, under all circumstances. I've got a board design that's used in production that the moment w

Because you don't know what the final design spec might need to be, under all circumstances. I've got a board design that's used in production that the moment where - depending on configuration - it could blow the device it's connected to up in about three seconds if you set it to full power in normal operation. It's designed to go that high, because it may be running from a very degraded power supply and it may be necessary to "kick" the actuator it controls quite hard with short bursts of full power to unstick it as it wears.