Why should I use ProtonMail instead of Gmail?
1. ProtonMail provides strong end-to-end encryption, making it so that only you and the recipient can read your emails. While Gmail also offers basic encryption, it only works when the recipient uses the same type of encryption.  Protonmail is also neutral (protected by strict Swiss privacy laws), has greater respect of your privacy and Protonmail is independent of the wider business model of Alphabet Inc. and their interest in data collection.
How does ProtonMail make money?
ProtonMail manages to produce revenue by providing paid plans for individuals or businesses, depending on their needs. The current subscriptions available are: 

- Mail Plus: Tailored for individuals, this plan provides 15GB of data, unlimited folders, labels and filters, unlimited messages, 10 different emails, custom email domain support, and access to the Proton Calendar.

- Proton Unlimited: This plan gives acces to 500GB of data, and with the benefits of Mail Plus, also gives access to Proton Drive, and Proton VPN.

Besides those subscription models, ProtonMail provides Business-to-Business plans for companies.

Overall, ProtonMail does not follow the expected model of selling user data, or providing advertisements. It solely relies on the users to pay for extra comfort on the platform.