Why do the function must pass event.EventData object

From this document

I found event.EventData object must be pass between notify and callbackFunction

The document give an example code use event.EventData object "evtdata"

evtdata = ToggleEventData(newState);
notify(obj,'ToggledState',evtdata);

evtdata is a ToggleEventData's object! I think pass 'evtdata' between notify and callbackFunction is enough.It seems unnecessary to pass 'ToggledState'.

I think both notify and callbackFunction should know evtdata is a ToggleEventData class object！After all we can type class(evtdata) to know evtdata is a ToggleEventData class,Matlab must know, callbackFunction must know it too！

Just like everyone knows cows are mammals