My understanding of why a shallow plane is good is that, with a shallow plane, your body is required to square the club (as opposed to your hands and wrists) and the face stays square longer before and after the hitting area, leading to straighter shots and smaller misses like small pulls and blocks.
In contrast, a steep plane cannot be squared with the body, but rather needs to be squared with a flip of the hands through the hitting area and actually a stalling of the body, which means your timing with the wrists and hands has to be perfect to hit a good shot and your misses get way bigger, like snap hooks and huge blocks.
It’s a good question though, and I look forward to what others have to say. I’m like you: I want to know why some move is better than another, and not just blindly do what others are doing without knowing why.