2. More efficient muscle use, less chance of repetitive stress injury. Regular cycling shoes may give you some lateral float, but they lock your foot to the pedal (fore-and-aft wise) in one place, and that's not how we use our feet. When you go up stairs or do leg presses at the gym (efforts not unlike pedaling up a hill), you push with the middle of your foot. Not with the ball of your foot, as you've been told is proper for cycling.

When you run fast, you run on your toes (or off the ball of your foot). When you walk, you land on your heel. Middle-distance runners run off their mid-foot.

Your foot is just a foot, but you use it different ways for different kinds of efforts, and click-in cycling shoes don't let you do that.

On long grinding hills, it is absolutely more comfortable to pedal close to your arch. You can't do that if you're clicked in. And on longer rides, it's good to vary your foot's position over the pedal, because doing this calls on certain muscles in your legs, and puts others to rest.

If your foot is locked in one position, you're much more likely to get a repetitive stress injury, for the simple reason that you repeat the same motion over and over.