Corner chairs, usually called roundabout chairs in their day, were occasional chairs often used in a corner or at a desk. They were not terribly rare—you can find antique examples at many decorative arts museums and in period houses where they are usually found in bed chambers, sitting rooms, dining rooms, or libraries. They were also called smoking chairs, barber’s chairs, writing chairs, and desk chairs, suggesting that men were the primary user. I found two portraits where the subject is sitting in a roundabout chair, and both are men. (See portrait of George Wyllis owned by the Connecticut Historical Society and the one pictured here of John Bours from the Worcester Art Museum.)
If you come across a roundabout chair with a particularly deep seat rail, it was probably used as a commode chair (AKA night chair, necessary chair, or closestool) with a chamber pot fixed below the removable seat. The deep seat rail hides the chamber pot.
Like so many fashions, roundabout chairs first became popular in England in the early years of the 18th century and spread to the American colonies. Most of these chairs were made during the period from about 1730 to the 1790s, after which their popularity diminished to the point that there are almost none in the Federal style.
Several reenactors wrote to tell me that it was awkward sitting in a roundabout chair wearing a sword, which I can visualize, I think. The sword point (sheathed, of course) would hit the chair, where it would not do so in a traditional chair. Whatever, it’s irrelevant because men almost never wore swords indoors. During the 18th-c and afterward, the custom was to wear swords for battle or for parade, but not for social events like balls or dinner parties. So the roundabout chair was not invented to deal with this “problem.”