From RJI Collaboratory, where I'm contributing: Ads are not so good for cashflow (a finance term that basically means real money you receive and spend) because of a few factors: You get paid AFTER you’ve spent money to deliver the service, the money can be hard to collect and campaigns are easy to cancel. In sum, ad revenue comes in late and can be unreliable. You’ll want to explore other ways of bringing in income.
Direct selling such as subscriptions and e-commerce can help, beyond being an additional bit of money. Selling directly, subscriptions, products, and so on, can be a more reliable source of income over time. You often control more of the process, and have broader leverage in determining what to charge. Your products and services can be rare or unique -- something someone can get only through you. From a financial perspective, people pay upfront, before you deliver the good or service, so you have the cash to finance your operations.
Bullet points are in the piece linked above.