Parallel inverters work in unison but give more output current. 2 inverters can give twice as much current, 3 gives can give three times and so on. If you parallel all the unused inverters at the output you will get more output current capability. The extra output capability allows it to charge and discharge load capacitance faster (= squarer edges) and ensure it can fully drive a transistor B-E junction into saturation. You can then produce almost any voltage you want by using the transistor as a switch to ground with a pull-up resistor to the voltage you need.
As SunnySkyguy points out, the extra current as the device changes state will make good supply decoupling mandatory.
Brian.