Toothed wheels on TMR trucks/wagons

Having finally got around to unboxing some TMR sets that I bought a while ago, I notice for the first time that some of the trucks/wagons have got toothed running wheels, even though they aren't powering some 'feature' of the truck's load.  Examples are Lexi's tender (but not her open truck), and Nia's slate truck with the elephant in it (but not her open truck).

Two questions: When did this design change first occur? and anyone care to suggest Why?

(Apologies if this has come up before, but I've been out of the loop for a while. I did search).

[Super]

I don't know for sure but I do have a theory:

Due to the rolling stock of the TMR's being so flimsy, short and light weight coupled with all the roller coaster type sets, I believe the toothed wheels tend to help those stay on track especially going downhill and on curves. In other words they can track easier behind the engines.