IR Transmitter Tower exist in both USB version as well as Serial cable version.
Page 7 of instructions for one of the sets (3804) that was using USB version has following explanation:
The IR Tower establishes a wireless link between your computer and the
RCX. With the IR Tower, programs can be downloaded from computer to
RCX. These programs ...
LEGO doesn't make something to directly facilitate this type of integration as far as I know. If you can get a PF extension wire, you could cut it and directly make the connections to your driver circuit.
You can drive the motor using the middle wires of the PF cable (C1 and C2).
For what its worth, I would personally prefer an H-bridge such as this one ...
Why they exist: to transmit programs from your computer to the Mindstorms smart brick (which is really another, probably smaller, computer). This was before USB ubiquity, and it also prevented children from plugging and unplugging a cable at the back of their computer, which is a dangerous move, even for adults.