They are completely different though they are connecting each other now.
Localization flow using machine translation is….
We choose a machine translation engine (could be BeGlobal or Microsoft Hub), then train the engine with related projects legacy. Its output will be tamed from this train and more appropriate if you compare with outputs from NON-trained engine. After applying MT, translators will do “Post Edit”.

On the other hand, localization flow using CAT Tool is….
First, we create a database (translation memory) which includes source & target pairs. Once the source sentence is updated or you want to translate similar sentence, CAT tool compares (leverages) the new sentence with the existing pair and shows us the difference between old and updated sentences. The difference should be translated BY HUMAN.
On either case, final process should be done BY HUMAN if you request premium quality.