Rich Notes is designed to be useful app with no purchase required. Casual users can utilize the base fonts, formatting, iCloud sync, passcode, and other features at no charge. This allows you to try the app risk-free.
If you find you like the app, then you have the option to invest in expanding how it can be used. In other words, try-before-you-buy. I've taken particular care to point out in the descriptions which add-on features require In-App Purchase.
Finally, I'm an indie developer (i.e. just a guy and his dog) with bills to pay like most people. Apps cost me time and money to develop. Your purchase allows me to build new apps, add features to existing apps, and provide ongoing support.