Only operations that change the state of the Ethereum network require gas. You can use an application and as long as you only perform read operations, you don't

Only operations that change the state of the Ethereum network require gas. You can use an application and as long as you only perform read operations, you don't require any gas. For example, in a decentralized Youtube-like application, you can browse and watch videos without using any gas. You would only need gas if you wanted to add content to the system, like uploading a video or posting a comment. If such an application required no cost to write data, what would protect the system against spam? Who would bear the costs of operating the application?