This is marked as a "double pole" switch, and the terminals are marked "supply L&N" and "load L&N".
I think, with a little guessing, the switch is designed to isolate both the live and neutral sides of a device from power. What kind of device, and why, is not clear to me. If you know any local experienced electricians you could ask them how they've seen this kind of switch used in practice.
But the way it appears to be used in your house is to switch two different lighting circuits. You have two switch loops there, operated together by one switch. I can only guess why this would be done, rather than using a more typical pair of switches:
- Maybe the two circuits are on different breakers so cannot share a live wire.
- Maybe one switch loop controls the bathroom light, and the other the fan. They are on two loops because that's how they were installed. They were combined onto a double pole switch because that helps prevent users from using the bathroom without the fan, and from leaving the fan running after leaving.
- maybe the installer only had one of these switches in his van. That's often the answer to many mysteries, but this would be an odd thing to have bouncing around in your van.
You could disconnect each blue wire, one at a time, to see what the two loops each control and whether they are on the same breaker circuit.