A trigger action can be used to combine different key strokes in one. For example, a counter has two traits, move forward one hex and turn. A trigger command can combine these two and the counter can move forward and turn by one key stroke.
Another use of triggers is to unify the menu listing on a counter. In Vassal it is crucial that traits follow a certain order, e.g. a label listed above a layer behaves differently from a layer above a label. If, for whatever reason, the order of the commands appearing in counter on right-clicks, a trigger can be used. Simply arrange the trigger traits in the needed order and the order of the original trait does not matter anymore. When the command name is removed from the original traits (only key strokes stay defined), only the trigger traits will be listed in the menu.
A trigger comes in handy when an alias is needed. For example a unit should move and turn by cursors keys and first person shooter style keys. One trigger reacting on cursor-up and one trigger for ctrl-w will do the trick.