You dont need the act but having the act you can simulate the dawn/dusk effect where the act come on before and turn off after the MH
having an act option is very nice for viewing the tank because things just look so cool.
I also have my act to come on around 1hr before and after the MH
A lot of people say the sudden light change from on to off or vice versa might spook the fishs causing them to "carpet surf".
Just wanted to add that act03 gives off a more purple color and the other is more blue.