You mean tools need justification? I thought if somebody had to go to the trouble of inventing and selling them that was justification enough!
Maybe : You're trying to tighten a nut the last 10 degrees, but tight access means you can't swing a normal wrench handle, or get a socket on it. So you need the vernier action of a ratchet handle, plugged into the offset opening of a crow's foot.
Alternatively, you need to accurately torque a nut you can't put a socket on, such as a fluid coupling. Just have to be careful to compensate for the lever arm length affecting the torque value.
Lastly, there are plenty of times you need a PAIR of 18mm, or 22mm, or somesuch, wrenches. The crows foot set lets you carry a SECOND, or SPARE set of the same size wrenches without the weight penalty of an entire second set. That's the ticket! Weight reduction!
Of course for the ultimate weight reduction, you should just carry ONE adjustable crows foot and a ratchet driver: