Gauges give you a nominal position. A point where the needle rests during normal activity. You can quickly scan the needles and only have to care if their position is out of place. Likewise scale, you don't have to calculate if it's beyond the reasonably expected level, you can simply see where it falls on the dial. With digits you have to examine each of them with specific knowledge of what those numbers mean. That takes valuable attention away from the more immediate tasks at hand.