I have about 20 total actions defined. Some I use nearly every time, and some I use rarely. I usually have 6 to 8 actions selected (checked) at all times and make only small changes to which actions are selected, depending on the circumstances. I use these almost always to modify full albums of music tracks.
It would be very convenient to be able to run only one of my predefined actions, without having to uncheck and then later recheck my normal group of 6-8. This would in particlar be convenient when running some of the seldom-used actions.
The user interface might either be another Action button that allows you to select exactly one of the predefined actions, or else add buttons to the current selection dialog, maybe next to the checkbox, that would let you run only that single action.