Because the output is open collector so it can only sink current. For this reason, the external pull up resistor is required for a current source as well as to pull the output high when the output is switched from the low state.
Some open collector outputs allow you to run the output stage at a higher voltage than the rest of the IC runs at. For example, you could run the chip at 5 volts and have the output switch 12 volts.
Open collectors are nice for "wired OR" logic. You can tie a bunch of open collector comparators together with only one pullup resistor.