The legend I've heard is that it was originally done so that if you were forced to make what we would now call an unplanned diversion (eg shot down by red triplane), you could sort yourself out with stickies and maybe even transport home.

What's this? The RAF have a "tradition"! Is this a first?

Probably started as a 'tip' to pay for cleaning-out the balloon basket.

ruff!!!
lsh