In many countries, the ones the ATM shoves out at you are far too large a denomination for everyday use. I found this to be an issue in Bangladesh, Vietnam and India for a start. Many banks will change these for no fee, so shop around if the first one you go to tries to charge you. Better to have a brick of small change than have to overpay every rickshaw driver who, funnily enough, doesn’t have enough change and has to force himself to accept a huge tip.