E.g. Germany is the largest net contributor to the EU, but guess what would happen if Germany decided to stop putting out those few billions a year. Yeah, not so good for the economy (not just of Germany, pretty much everyone). Turns out, those few billions spurred trade much greater than that. It's literally "one stick, alone, weak — many sticks, together, stronk". Just that the bundle is stronger than its constituents.
(I've heard the UK is working earnestly on an experiment confirming this.)