MIDI cables are not just for
sound cards -- they allow you to
connect any two MIDI devices, provided one has a
MIDI Out or
MIDI Through port, and the other has a
MIDI In port.
For example, you might use a MIDI cable to connect the MIDI Out port of your MIDI keyboard to the MIDI In port of a separate tone module, in order to use the keyboard to trigger sounds in the tone module.
Another thing you can do with MIDI cables is called "daisy-chaining". Here's an example:
You might use one MIDI cable to connect the MIDI Out of a hardware sequencer to the MIDI In of a synthesizer, and another to connect the MIDI Through of the synthesizer to the MIDI In of another synth, and a third to connect the MIDI Through of the second synth to the MIDI In of a drum machine, etc. This is a method that allows all the "downstream" MIDI devices to receive input from the sequencer.