You are right, one in digital IO, and the other is analog. Out of these two ICs, I have only used the PCF8574, so the following about the PCF8591 is only based on what I know of the IC, not from personal experimentation with it.
- Write the lines low to sink current. << Drive LEDs etc. (BTW, it can sink, but not source current)
- Write the lines high (float the IOs, but with pullups) and then read the IOs. << Good for reading switches etc.
- Read analog values (like from sensors). You could connect a POT, photoresistor/phototransistor, thermistor, etc.
- Write analog value. This is only one channel. I can't think of a good beginners example for this.
I'm all for gun control... that's why I use both hands when shooting