Think about your standard editing software. It has a function to read from its data source (wherever that is), a function to write to the data source, and functions to change the content. As a piece of software on your computer, all of these functions can exist in the same package. You’re generally only telling it to process one file at a time. However, if you put your software on a web site, it stops making sense to have all of those functions in one place. Developers use tools like Firebase to hold their reading and writing functions in one place. They can then write their web app code with a function to make requests to the Firebase. This allows a single backend process to respond to the requests from many frontend users and this is the basic use of modular web services.