The right lane is for passing and slower traffic should keep left. It's one of the first things you learn when you get your driver's licence, yet so many drivers forget it or don't understand it. Here's a detailed explanation for why this simple rule is so important.
This video, from the Vox YouTube channel, explains why driving in the right lane is not only more dangerous for you, but more dangerous for others around you in addition to making traffic much worse. [Note: The video was made in the US, so the lanes are in reverse. Nevertheless, the principle remains the same everywhere.]
When slow drivers are in both the right and left lanes of a highway, faster drivers have to weave back and forth to maintain their speed. And as it turns out, the biggest predictor of an accident is actually a driver's variance from the average speed of traffic around it, not speeding. That means all that slowing down, weaving between lanes, and speeding up by fast drivers to get around slow drivers is what causes the most car accidents.
In fact, a car in the right lane going 8km/h slower than the flow of traffic is far more likely to cause an accident than a car going 8km/h faster — even if the slower car is already going the speed limit. That's why we have a law delegating right lanes to passing only. So if you get a ticket for hogging the right lane, now you know why.