Your coworker paid for your lunch? A friend paid your cocktail? Don't forget about it and let them know that you remember!
Just enter the amount and the other persons e-mail address to save the owe. If you like, you can also add a subject so that you know why you owe someone when looking at it later.
Whenever you add a new owe, the other person is informed about this via e-mail. They know that you will remember and can look it up any time.
Balances between two people are calculated based on all their owes. This makes exchanging money unnecessary in many cases when you pay for each other frequently.