[kalleboo]

Super simplified (and not real) example.

Say it's a super sunny day, Germany generates more solar power than they use themselves (Germans don't usually have aircon at home anyway), Poland is happy to import and use it since it's basically free since it has no input costs of fuel, so they can idle their coal plants and use less fuel. Now it turns to night and the sun goes down and Germany generates 0 solar power. Then they can import coal power from Poland.

Expand this over a whole content and you get flows of electricity between companies depending on how the grid interconnect loads look and the prices of the various power sources. If Polish coal costs more then French nuclear, then Germany can sell their cheap wind power to Poland for something in between polish coal costs and French nuclear costs, and then buy nuclear from France and pocket the difference. Assuming the grid interconnects can't handle Poland importing direct from France.

Again none of these examples are real, just an explanation of why you would want to trade electricity and why you have a big market like the EU power market.