Zwammerdam 2nd century boots

[Doc]

I suppose that there are two reasons the same of which are used for caligae.

One reason is to hold the upper to the sole. Stitching is not always strong enough to hold everything together. The other is, like you pointed out, the sole would not really be able to take constant walking on pavement, terrain, etc. Thus the nails are used to protect the sole by keeping it out of direct contact with the pavement. There is also the issue that the nails provide good traction especially in non paved areas. It is my understanding with calige that once the nails were worn out, the caligae were replaced not the nails.

Maybe it is found on women or children who lived, worked, or carried out activities in areas where the ground required such footwear?

Also children destroy and wear shoes out quite readily especially at the soles. When I was a kid, the sole and the edges where the sole met the upper was always were the first to go. Maybe the nails were necessary to make kids shoes last longer; running around playing, working with the father etc......