Many dummy tanks were built, some for deception purposes, others for war bonds or recruitment drives. I doubt whether there were very many that were as big as the monster that was used in Hobart, Tasmania that can be seen in the picture made available by the AWM (Photo number H16150).
I cannot access the picture, but could it be that the dummy was simply a unit for observation from aircraft, to train aircraft crews and such?
One could get more hypothetical and think it was a form of psychological warfare. If you put out giant dummy tanks, recon aircraft would see them, and assume the rest of the landscape was on a similar scale, they would think they were closer to the ground, thus they might not see the little infantrymen running around on th at all, as they would be vastly smaller than the giant dummy tanks. I'm not sure if such deceptions have ever been carried out though.
This one was used for recruitment in Tasmania, Australia. Judging by the length of it, there was either a second vehicle under the tank (unlikely) or it had a set of wheels or rollers at the rear end of the tank because it's way too long for one vehicle to carry. To search for the pictures by number at the AWM, click on "Search our collections" and use the photo number as the search term. I've attached the Tasmanian monster here.