[1] The device was meant to address a limitation in the built-in game control offered by the Apple II, by allowing either four Apple-compatible paddles or two Atari CX40 joysticks (but not both types at once)[2] to be read by the computer simultaneously.
The recommended Atari joysticks are switch-driven (i.e. digital), instead of the usual smoother-action analog sticks available for the Apple II.
[3] Since the Apple II hardware makes no distinction between two paddles or a single analog joystick plugged into the same jack,[4] it is also been possible to connect and read two fully analog joysticks with the Joyport via the paddle jacks, but few (if any) two-joystick games supports this, and Sirius did not suggest it.
Why not is unclear, but there may be a noticeable speed advantage when driving two digital rather than analog joysticks on the limited hardware of the time.
Also included are BASIC and Pascal source code listings of sample programs making use of the Joyport, indicating the hobbyist influence in the market at the time it was released.