Green baggage tag to non-EU destination

It's indeed kind of pointless if you are leaving the EU… but so would be devising a system to tag luggage according to their destination, complete with two types of labels, a system to handle layovers, etc. Basically, green tags are not for travel withing the EU or anything like that, it's for luggage originating in the EU.

That way everything is tagged very easily, merely by virtue of being checked in at an EU airport and it's sure to be correctly tagged whenever you travel within the EU. You don't need to designate some check-in desks for intra-EU travel, interface with existing systems, train employees to handle EU luggage differently or add any logic or software at all. You put paper with a green stripe in the printer and you're done with it. That's simple and efficient and good enough for its purpose.

Incidentally, you can easily have non-green-tagged luggage on an intra-EU flight, for example if that flight is the last leg following an intercontinental flight. It does not really matter whether you go through several EU and non-EU airports, take a domestic flight, an intra-EU flight, or even transit outside the EU between two EU airports; your bag will have the right tags regardless.

This largely covers it. Notably, all flights from the EU have their baggage tags printed with a green-edged label.

The point of them is explained clearly there:

In large airports, it is not always possible to separate travellers flows according to the point of departure of their travel, whether they started their journey in an EU or in a non-EU airport. It can therefore happen that travellers of both categories mix in airports common areas. However, luggage which come from a non-EU airport can be subject to controls while luggage registered in an EU airport do not (note 2).

Consequently, in order to allow for a quick and effective identification of luggage according to the airport where they were checked in, and, thereby, to optimise controls on luggage originating in a third country while ensuring freedom of movement of the other luggage, the latter receive a green-edged label the model of which is different from those used elsewhere than in the EU.

The "controls on luggage" primarily mean customs. The tags both provide some means to filter what bags customs can search at random, along with some means of checking only those entitled to use the blue "arrivals from the EU" channel at customs are.

So, they're essentially meaningless if your destination is outside the EU; however, if you have a layover outside the EU and then return to it (e.g., Paris–Zürich–Rome) without having access to your baggage then they do have meaning despite having left the EU in transit.