Why do French drivers keep their blinkers on in the left lane on motorways?

What is it supposed to mean? It only happens in the innermost lane. In Germany, for instance, it means 'move to the right, I want to pass you,' and the car quickly overtakes. Here, when I move to the right or middle lane, nothing happens—they keep driving at the same speed without even trying to overtake. So, after a while, I end up getting back in front of them to pass slower cars.