Two character route codes used to be a very good way of conveying to passengers a lot of information very quickly as trains arrived. People knew their numerical codes (e.g. embarking at Wimbledon for Kingston could be 21 or 24). It was extremely rare for these codes to be wrong. On 4SUBs the driver had to open his front cab window, lean out and drop the stencils in to the frame. In order to avoid having to carry more than necessary, SWD route codes had no repeat numbers - i.e. 22, 33 etc.
Today, when the provision of this information should, and could, be as good or better, it is not, because; 1) the destination indicators are very often almost useless on the trains (700,707, etc), 2) the detailed routeing information that used to contained in a numerical code has been lost when spelling out a destination alone, 3) the platform indicators are hugely better today, BUT are far too often wrong or badly-sited (Clapham Junction p15 is staggeringly poor, for example).
Some units still do occasionally display two, or even four-character codes when certain drivers programme them accordingly. SWR 455s have (or had until recently) blinds showing destination and code, but the numbers are so small as to be almost useless. See attached shots (the 1L98 'Desiro' one was on a Portsmouth-bound train at Woking!):