What is the role of and why are side effects used in programming?

Side effects in programming are any changes in state that occur during the execution of a function or method, besides returning a value. For example, changing global variables, writing to a file, outputting to the screen, or changing UI state.

The role of side effects:

• Allow interaction with the external world (e.g., saving data, displaying information).
• Are necessary for changing the application's state, such as updating the interface in Android.

However, side effects complicate testing and debugging, so modern approaches aim to minimize or isolate them.

Example: in Android, when a button is pressed, the text on the screen changes — this is a side effect because it changes the UI state.