I am studying IO Expander (NXP, PCF8575) using I2C. Most IO Expander have interrupt pins to notify the master device of changes in the input.

So I want to know if it is mandatory to connect the interrupt pin to the master device.

 

No! but it depends on your personal requirement. If you need the device to tell you something has changed use it, or if you only need to poll the device then don't use it. If you use it for output! well then again no. If you are time dependent and need to know some pin has changed then yes you need it!