The app exists for more than 6 years and the premium version allows me to keep improving it. Additionally there are running costs that need to be covered such as server costs, Apple developer account costs, getting current currency exchange rates and many more.

I do not like ads and their user tracking in apps, so I do not use them in my app. As alternative power users need to pay a small amount to use the app with unlimited entries and all premium features. If you just want to keep track of a few debts you should be able to use the app completely free.

You can choose between a subscription (monthly or yearly) and a one time payment. The features you get are the same it is just a difference in the payment method.