To connect Sunbury, a part of the Greater London area, to the city centre with more affordable travel costs. Many people commute from Sunbury into London and despite being much closer to the city centre than many other zone 6 stations, they are forced to incur far greater travel costs due to being one station stop outside of the Oyster network. Trains going through Sunbury also only go to either London Waterloo or two stops further out to Shepperton. Therefore it seems illogical for it to remain outside the Oyster network.