There was a number of reasons. For example article 270 of the treaty states that

The Allied and Associated Powers reserve the right to apply to German territory occupied by their troops a special customs regime as regards imports and exports, in the event of such a measure being necessary in their opinion in order to safeguard the economic interests of the population of these territories.

The occupied territory in question being the Rhineland. Alan Sharp says that this was used as a means of economic control over this heavily industrialised region that would allow the entente to penalise the German government for non-compliance by setting up trade restrictions between the occupied territory and the rest of Germany. The logic would also follow that also as a valuable industrial area it would be useful to hold in the event of another war.

Concerning the Maginot line its useful to bare in mind that the original plan for the line included it being extend into Belgium running along the Albert Canal and the River Meuse. The plan was part of a Franco-Belgium alliance that would also Anglo-French troops to be deployed along the fortifications in Belgium well in advance of any attack. However in 1936 after the reoccupation of the Rhineland by Germany Belgium lost its confidence in the Anglo-French willingness to take action in the event of a conflict and as such decided to return to a policy of neutrality. The defences weren't as well built along the rivers as originally intended and the Beglium army wasn't large enough to properly occupy them. As a result when Germany invade in 1940 they managed to breach the defensive line before French and British forces could get to them, that combined with the attack through the Ardennes caused the collapse of that flank and the ultimate Nazi victory in the Battles of France and Belgium.

And while occupation of the Rhineland may have been seen as valuable to the contianment of Germany in 1918 I'd point out it wasn't for another decade that the Maginot line's construction began.