Travelling to London in the morning boarding after Reading meant there was little or no chance of getting a seat, unless in First class.
Coming out from London you could either travel off peak or run with the other commuters from the lawn to get that prized favourite seat.
So the concept of a season ticket with First class one way and Standard class the other was born, or at least so the tale goes.
Whether it will ever be extended to Reading and the West is an interesting question, at the moment I believe it starts from Twyford?