TLDR; We tend to value large bills over the same amount of money in smaller denominations.
Studies show that we are less likely to break a large bill for small purchases. On the other hand, if we have coins or smaller denominations it's more likely that we'll spend them.
If you're on a budget this tendency is something to be aware of. Those small purchases can add up quickly. This effect can also be seen in credit card spending. Spending with a credit card eliminates the denomination altogether and so the reluctance to break a bill goes away entirely. This could be part of the reason so many people go into debt so quickly with credit cards.