They were Siege Guns. No "propaganda".
I'm sure there must have been SOME ONE on Hitler's board that noticed "Hey, you can just make 5 Tanks instead of this thing". Hitler must have knew that, he just wanted it anyway to show his miltary's strength. It's a drive weapon, I know, but for an army that was based on fast moving blitzkrieg tactics, I can't imagine it being built for any other reason but propaganda.