Round female pin header vs square female pin header
The round pin headers (also known as &quot;machine pin headers&quot;) typically provide a more secure fit, with more contact surface area between the pins and sockets. They are often used in applications where vibration is a problem.
They're not used universally, because the machining process makes them significantly more expensive to manufacture than the square pins. Also note that the square pin typically won't fit into a round socket, and round pins going into square sockets can have problems maintaining position (or even contact).