It’s an Eclipse Smarthome concept: smarthome/docs/documentation/development/bindings/thing-definition.md at master · eclipse-archived/smarthome · GitHub (see trigger channel section)
Unlike other channel which can be bound to an item, trigger channels do not have a state. There is just an event (button pressed). If you have a channel which is bound to a Switch you have an On/Off-State whch is just artifical as the button is not on or off at all.
Moreover, using a Switch would mean that it’s possible to change the state of the item, but for a simple button you just cannot set/change any state.