I have picked up several* carbon steel twist bits over the years that would have been used for drilling metal. No doubt not an easy task, but the torque a brace offers would have its benefits. I suspect this would have been an apprentice job: "Just lean hard on the tool and keep turning, lad. I'll be back to let you know when it's time for lunch."
Interestingly, though, I also have a breast drill with a two-jaw chuck for tapered shanks. This makes a lot of sense, since it's hard to get the a hand-tightened three-jaw chuck tight enough on large bits, at least for me; they invariably spin in the chuck when the going gets heavy. So it's possible these bits could have been used in a breast drill rather than a brace. But, given that I've seen just one breast drill with brace-type jaws - and that one a late Stanley (based on the color), I think a lot of braces got used to grunt holes through iron and steel.
*Well...Dictionary.com defines "several" as "more than two but fewer than many." So it's possible, just barely possible, that I'm understating how many I have.