Coins are easier to count than bills. They are thick enough that you never confuse two coins for one, even when they are stuck together. And they are also more convenient in your pocket, because you don't have to fold or otherwise organize them.

Dollar bills are usually in terrible condition. Folded corners, creases, dirt. Ten singles take up more space in my wallet than just about anything else I'd put in there.

I'd rather have ten coins. They'll easily fit in the bottom of my pocket, and when I pull out change there's likely to be a useful amount of money in it.