Hello Gaz, I think it's tradition and is always used as the players run onto the pitch pre-game. It relates back to the Charlton nickname of 'Robins' since the 1930s and there was also an association with the crest before it was changed to the sword (though the club are known more by the nickname the Add1cks nowadays). Billy Cotton recorded a version of the song for Charlton after they won promotion to the then First Division in 1936 and he became a friend of the owners of the time, the Gliksten Brothers.