As explained in previous answers they are ‘primarily’ there to assist steering but there is a little more to it than this. read more
On the battlefield the wheels clogged up with mud and would have required a lot of maintenance, so it was decided they were more hassle than they were worth. The Mk I tanks still in use had their steering tails removed and further marks never received them, although it seems a prototype Whippet was given a set, and either prototype or early production Gun Carriers Mark 1. read more