Why do developers use "fake data"?

When making applications that would uses data, developers often use "fake data" as a stand in for real data so that they can check that their code can properly use the future real data. When making an application that has to deal with user data in order to do something like, remembering a user's birthday to give them a free treat. In order to know that this feature can work in the first place developers would make up "fake data" in order to see if their code can do this for the real thing. Using fake data is just a smart way for developers to make sure that their code can properly use the data they are getting from their servers.