If users want to use actions they have to click on an entry in the menu. It would make actions more flexible if actions would be triggered by events, e.g. a users enters text in the message field or the user presses some keys as a shortcut for an action. The events are known well by programmers (Keypress, Focus etc.). That way actions in Claws Mail would be more powerful and users still don't have to start to programming for Claws Mail to use this power.
You probably missed one possibility - menu entry can have an accelerator assigned, so the action is performed on some user defined keypress. Also, an action could be assigned to a user defined toolbar entry, it is performed with a mouse click. More elaborate actions are available via some plugin, especially scripting plugin - there are Perl (somewhat limited to filtering), Python (extending the basic functionality with definition of various new actions with ability to run some scripts on specific events) in official repository plus not yet official Lua plugin available on request (ported Python plugin's methods/properties with some extensions). As actions are usually shell scripts, wiring them to such low level events as noted (Keypress, Focus...) could lead not to the best user experience... (my oppinion, others may see it differently). But still, there is much to explore...