Spoiler alert: it's not about the hoof mechanism
It's a well documented fact that a horse's hooves will expand when loaded and contract when lifted off the ground. It is also well documented that nailed horseshoes can impair this ability to varying degrees. Proponents of barefoot horse keeping and farriery traditionalists will argue at length as to how much a properly fitted horseshoes will inhibit the so-called hoof mechanism.
But according to Professor Robert Bowker, the hoof mechanism is not the problem if your horse is shod. It's a bit more complicated and a lot more nuanced than that. In this feature, you can also learn how to help your shod horse by not cleaning out his feet all the time.