Sometimes a chip or a product is designed to interface with a variety of different equipment that use a range of logic-1 voltage levels. If the device in question always outputted a logic-1 of 5 volts then it might damage a device connected to it that requires a logic-1 level of 3.3 volts. There's no risk of damage should the output be purely open-collector (or drain) but, it requires a pull-up resistor to be fitted for it to work.

Another reason is that you can tie several open-collector (or drain) outputs together to form a wired-OR logic i.e. any of the the transistors operating will pull down the pull-up resistor thus you get extra logic functionality for the addition of just one resistor.

Sometimes a device is designed to interface with a variety of different equipment that may use a range of logic-1 input voltage levels. If the outputting device always outputted a logic-1 of 5 volts then it might damage equipment connected to it that requires a logic-1 level of 3.3 volts. There's no risk of damage should the output be purely open-collector (or drain) but, it requires a pull-up resistor to be fitted for it to work.

Another reason is that you can tie several open-collector (or drain) outputs together to form a wired-OR logic i.e. any of the the transistors operating will pull down the pull-up resistor thus you get extra logic functionality for the addition of just one resistor.

Sometimes a chip or a product is designed to interface with a variety of different equipment that use a range of logic-1 voltage levels. If the device in question always outputted a logic-1 of 5 volts then it might damage a device connected to it that requires a logic-1 level of 3.3 volts.

Another reason is that you can tie several open-collector (or drain) outputs together to form a wire-OR logic i.e. any of the the transistors operating will pull down the pull-up resistor thus you get extra logic functionality for the addition of just one resistor.

Sometimes a chip or a product is designed to interface with a variety of different equipment that use a range of logic-1 voltage levels. If the device in question always outputted a logic-1 of 5 volts then it might damage a device connected to it that requires a logic-1 level of 3.3 volts. There's no risk of damage should the output be purely open-collector (or drain) but, it requires a pull-up resistor to be fitted for it to work.

Another reason is that you can tie several open-collector (or drain) outputs together to form a wired-OR logic i.e. any of the the transistors operating will pull down the pull-up resistor thus you get extra logic functionality for the addition of just one resistor.