It actually worked too

No, they did this to make the Germans think they were going to invade Sardinia instead of Sicily.

Allies: *creates a bunch of fake armies made of cardboard, inflatable balloons, etc to redirect their attention on a possible invasion zone*
Germans: We've been tricked, we've been backstabbed, and we've been quite possibly, bamboozled

If I remember correctly there was much more to it than that. Fake secret messages that'd be intercepted by the Germans, civilians helping the spies and establishing their own networks, etc.