There are oils for Euro cars, because most cars in Europe are turbo diesel. Diesel fuel is much cheaper there than gasoline and turbo diesel engine have better MPG than gasoline engines too. So a lot of people drive those cars thar are mostly German - VAG (VW/Audi Group), MB, BMW.
Turbo cars need more robust oils and the diesels are high compression engines too. There are significant amount of diesel direct injection too.
ACEA C2/C3 oils are designed to protect the diesel DPF system and the cat. converters too, but they are suitable for gasoline engines too.
ACEA A3/B4 are oils designed mostly for turbo gasoline cars like Porsche, VAG, MB, BMW and turbo diesel cars without DPF. For some reason the turbo German cars always run hotter by default (especially the VAG cars) and for that reason why require more robust oil. The Porsches have rear or mid engine and also struggle with higher temperatures than usual.
Euro emission standards may be a bit stringent than US, but they really don't have that MPG resource conserving thing that EPA enforces here. Or at least I'm not aware.
So people there run ticker oils with higher HTHS values that maybe a bit less efficient for the MPG, but provides much better lubricity protection. That's all.
By using those oils you just ad extra layer of protection to your engine and probably a bit extended OCI. However, some thicker/higher viscosity oils may affect the lubricity of the cam chain in the winter.