As with most commuter TOCs, the primary reason to get First Class is so that you are more or less guaranteed of a seat at peak times. You might even get two seats to yourself. In the declassified First at the back, unless you get one of the two single seats, you are bound to be sat next to someone. Also, the rear declassified First is usually full at peak times once it has stopped at busy stations. For example, in the morning peak, Gatwick to St Pancras will have already been filled by commuters from stations further south on trains from Brighton or Littlehampton. In the evening peak, St Pancras to Gatwick should be fairly empty on boarding, but it will be full by Blackfriars.