The Piko starter set I was sent for review comes with it's own transformer and controller tailored to the voltage and current demands of what is a far bigger and heavier model than the 4mm scale stuff I'm used to. The controller is of the centre off variety, turning the knob one way or the other from twelve o'clock changes direction as well as speed. Though my first train set controller, dating back to the sixties, has this arrangement, it's 'feel' was a bit alien to me. For the last thirty or more years I've used a series of controllers that have a switch to change direction, the rotation of the knob solely governing speed.
It seemed till recently that the centre off pattern of controllers were synonymous with train sets; I began to wonder why this might be. I doubt that there's more than pennies to be saved over the separate reversing switch, but maybe with one controller used in a range of trainsets and remaining in production for many years those savings were well worth having. Perhaps it was thought easier for young minds to understand, turn it one way it goes that way, turn it the other and off it goes in reverse.
The direction switch model should give finer control, having double the distance to sweep from off the maximum speed. I'm sure that this is why the serious enthusiast has adopted this pattern so wholeheartedly. But as ever there are exceptions, Bachmann bundle a very nice direction switch controller in with their trainsets and conversely some of those who supply what's thought of as the enthusiast market do or did centre off models.