What Was It For?

I did see a youtube video that had a rebuilt Mk IV being driven around at some kind of armor show in England, I think, and it did not appear to have one so you may be right.

I don't know much about WW1 vehicles, but i think those wheels were only fitted to the Mk I's. I had no idea why, but Steve's explanation makes sense.

The one in England is a replica at the Bovington Tank Museum. They also have a real one inside that you can go in.

According to a photo caption in The Tank Story, the first tank book I ever bought some 40 years ago (for the whopping sum of $1.95), the wheels were "an aid to steering".

In short, yes, it was to aid in steering.  It also slightly improved trench crossing abilities.  Little Willy (the Landship Commision's first prototype) had one, and the Mk I's had them.  However, they were found to be more of a hindrance than a help.  They were often the first things damaged during the battle of Flers-Courcellet.  A detachment of Mk I tanks served in the Gaza campain without the tail wheels, and the following Rhombiod tanks (Mk II through V) were not equiped with them.  

My next project is the Takom Mk. I, and I've been reading quite a bit about them.  There are some interesting accounts of their first deployments.  There were two tanks in particular, C5 - HMLS Creme de Minthe and C6 - HMLS Cordon Rouge, that made excellent impressions on their first day.  Creme de Minthe had half of its tail wheel shot off before wreaking havoc on the Germans at the Courcellet sugar refinery.  

You can see the steering tail doing its job in this video clip.

https://www.youtube.com/watch?v=b-9n3GUwIDo

The "MkIV" you mention is actually a full replica made from original drawings, not a rebuild.  A 1:1 scratchbuild!!  It belongs to the Bovington Tank Museum and was on display in London at an event to mark the Somme anniversary.  It gets regular runs out at the museum.  I believe the tail wheels on the MkI could be raised for skid turns.  As others have said, they proved useless as any kind of steering aid.

I'm not sure what the wheels were for on the WWI tanks but I do know that the very early crawler tractors did use them for steering. The Lombard steam tractors used in the New England forests used skis on the front to steer as did the later Linn gasoline tractors, much like a more modern halftrack. I also have seen pictures of Best tractors with them and there was a very early gas powered crawler crane in my hometown with wheels for steering.

Thanks for the info and the great video of the Creme de Menthe. I'd really like to know where you guys find all this info. Since I've never done armor before I'm limited to the internet (no books on armor in the house).

I know that video says that the Mk I is Creme De Menthe, but that's an error.  CDM was a "Male" tank, and the one in the video is a "Female."  

http://www.landships.info/landships/index.html

There's a ton of interesting stuff in the forums of this website.  It may take some searching, but there's good info.  

Books to check out are: 

The First Tank Crews, by Steven Pope

British Mark I Tank 1916, by David Fletcher, and 

British Battle Tanks WWI to 1939, by David Fletcher.  

The books by David Fletcher have the same material that cover the Mark I tank.  The first one will do fine, if you only want info on the the Mark I.  The second book would be better if you want info about the mark I and its successors.    

I like the Steven Pope book the best, but it is the most expensive.  

-Chris