system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is being flat-footed a hindrance?

As a child, I was flat-footed - my father insisted I needed orthopedic insoles at a young age, so as to be able to serve in the armed forces if my grades didn't come good. Lo and behold, I shit-talked my way into a decent paying desk job as a young adult that did not require a protractor to get me hired. So, what advantage does an arched foot have?
gpt
Feet are supposed to have an arch, and the bones/muscle in them require it for proper function.

It's a bit like a bridge. If you look at large bridges that support a lot of weight, you see those giant cables that 'curve' up and to a point in the middle. So when you put a lot of weight on the bridge, it's not just the actual bridge, road, and columns underneath that support that weight. It's all anchored to a particularly strong 'tower' and then redistributed across the weight of the whole bridge. This allows the bridge to sustain itself, and significant weight, without fear of collapse in any one part. 

The arch of your foot is basically like that. The bones in the middle connect all the muscles to an anchor point. So when you walk, and you roll your foot, the muscles have an anchor point that they can pull on. That anchor point then redistributes the force across the entire bone structure of the foot. It means that any individual spot on the foot can share the load of walking on other parts. 

When the arch collapses, there's less work-sharing going on, it means each piece has to work harder, and it can't 'flex' as well with other pieces. As a result, flat feet tend to get sore faster, have a varying limit to their range of 'motion', and are prone to pain under high workload, such as marathon running (or sprinting from zombies). 

It's notable, as well, that arched feet typically have a 'resting' position where the feet are not strictly pointing forward, but not pointing 'too out' either. This causes the foot to rest, or work, at a certain angle. The legs, hips, and back, all work together to keep you upright. When the feet are overpronation (tilted 'inwards') or supination (tilted 'outwards'), it forces the leg muscles and bones off kilter. This forces the hips out of alignment, and can even throw the back muscles off to some extent. 

It can cause various alignment issues all the way from the feet, up to the shoulders, in severe cases! This can be very uncomfortable. 

***The benefit of the arch, then, is that it helps the foot to spread the workload across a wider area. This, in turn, supports healthy muscle tissue, and reduces inflammation and stress on individual muscles. So an arched foot works less to do more, while keeping your body properly aligned!***