Many artists like to have a round seal, either to fit their design or as a nice contrast when used in conjunction with a square seal. You can buy round, cylindrical seal stones but I recommend carving round seals from square stones. My main reason is knowing which way is up. When you stamp a round stone it can be hard to be sure where the top of the design is and not have the seal impression angled in one direction or another. If you carve your round seal design into a square stone you’ll never have this problem. There is historical precedence for this too as early seal carvers did the same.
The illustration (pulled from the internet) is an old seal for a company that has a round seal carved from a square stone. Today most companies in China, as elsewhere, would just get a rubber stamp made instead.