Why did the French demand control of the Rhineland?
The French, it's safe to say, harbored a lot of hatred for the Germans. After all, most of the war on the Western Front had been fought on French territory, millions of French lives had been lost, and billions in property damage.
Still, France was an empire, and had to show something for the victory in terms of spoils for the war. So part of their rationale was to occupy a key portion of Germany's industry and resources, which the Rhineland was. Secondly, the Rhineland was, of course, right on the border between Germany and France, which gave them a sort of buffer zone against any future German attacks.
With the $40 billion in reparations Germany was supposed to pay the Allies after the war, France took some of their payments in material goods, forcibly taken from the Rhineland in the form of factories, trucks, machinery, and many other things that weren't nailed down. For all of these reasons, the Rhineland was important to France after the war.