They are just so disproportionately grateful when you do. It is as if you have saved the life of their firstborn or told them they have won the lottery. You can almost forgive them for walking too slowly in front of you with their Day-Glo rucksacks and taking ages to find the correct change when you are behind them in a queue to buy a newspaper.
I once told a man from Singapore which train station to get off at and he proceeded to take a photograph of me, note down my address and once he got back home he sent me a thank you card and a copy of the photo. Obviously slightly nutty but heart-warming nonetheless. A decade or so later, he probably still tells his neighbours about the lovely helpful man from England and shows them my photo on the mantelpiece. I wait to see if I am remembered in his will.