A well-known advertisement by American Express urged travelers returning home to keep their travelers checks rather than cash them in. The reason given was to have cash in an emergency, but could American Express have had any other reason for encouraging this behavior?

They certainly could. At any time, American Express is holding over $1 billion from clients who have purchased travellers checks but have not yet used them. American Express invests this money and earns a sizeable income on it. As long as their clients keep travellers checks outstanding, it is American Express and not the clients that earn interest on this money.