I have several pin vises, including one similar to the photo. I bought this style specifically for use in scratchbuilding. The hex design of the collet nuts facilitates making round tube or rod into hex tube (to make nuts) or hex rod. Chuck a piece of tube or rod in the pin vise, then clamp the pin vise horizontally into a bench vise, with vise jaws gripping the pin vise by the hex flats. File a flat on the top surface of your rod/tube material, then release the bench vise, turn the pin vise by one set of flats, and re-clamp in the bench vise. File another flat across the top surface of the rod/tube, release the bench vise, and repeat until all six sides of the rod/tube have been flattened.
For those who like to have several pin vises around, picking up one of this type may be useful. Mine has become the 'go to' pin vise on my bench (Note: Mine is not the brand pictured. The one I have looks exactly like the one sold by MicroMark).