A "normal" digital output actively drives both high and low. Open drain or open collector outputs only actively drive low, and go to high impedance when the signal is meant to be high. Some reasons the latter can be useful:
- Multiple outputs may be driving a bus. When devices only drive the bus low and a passive pullup drives it high, there are no electrical problems with collisions. The IIC bus is an example of this. Look at the protocol spec and see how the "anyone that drives it low wins" electrical result was used to advantage.
- The output can often go to a higher voltage than the supply when high. Since the only thing connected to the output is a collector or drain of a transistor, the voltage limit is whatever that transistor can withstand. With outputs that are also actively driven high, there is generally static discharge protection circuitry that clips the output to the supply voltage.