French warship designers of the late XIX and early XX centuries seemed to love the tumble-home design... I think they expected it to be more efficient at deflecting enemy shells than a vertical side. Here you have some pictures of the pre-dreadnought battleship Tsesarevich, from the Imperial Russian Navy, but designed and built in France:
Its commonly stated that the tumblehome was to help deflect shells up. This isn't the reason, or at least no the primary reason.
The limitations of the guns at the time (and the targeting apparatus) meant that the shells likely to hit the hull would be fired at fairly short range and so would have a fairly flat trajectory.
If you had, say, 2 inches of armour and vertical topsides, the shell had to penetrate 2 inches of armour. But if that armour was raked at 45 degrees, the shell had to travel further to penetrate the armour as its travelling at an angle through the armour- just over 2.8 inches. So raking the armour has the effect of increasing the thickness of the armour (from the point of view of the shell) without any weight penalty (a weight saving potentially, as for a given waterline beam you ended up with less deck, which is itself more weight).
Its the same reason as armoured vehicles tend to have raked sides. The rake may help deflect the shell up, but that's not the primary reason for the rake.