It never fails - every time I'm in a taxi, the final fare happens to be either 45 baht, or 65 baht (or something very close to these).  You can either fumble with a fistful of 20's - if you have enough - or give the driver 100 baht note, which are plentiful.  If you're lucky, you'll get the correct change back (sometimes a fistful of coins), but quite often, you'll get a grinning driver who tells you 'Sorry, no have change.'  As you dig in your pocket for more, tuk-tuks and taxis are honking behind you and motorbikes are whizzing by your door - it's just a crap scenario to be in.  This is all usually avoidable if you have a 50 baht note (sometimes you need a 20 to bolster the final tally), but they're not as common as you'd think!  That's why, in Thailand, the 50 baht note is one of my all-time underrated things.