Functions are composable. Your statement is not. If I would ask you to create a snippet that searches for cats, another one that searches for dogs and a third one that searches for staff that owns both a cat and a dog you will either duplicate a lot of code or end up with functions.
Now if you restrict yourself to not introduce any side effects, the interpreter can be proven correct, the execution can be analysed much better and you can have some neat safety guarantees.
Now if you restrict yourself to not introduce any side effects, the interpreter can be proven correct, the execution can be analysed much better and you can have some neat safety guarantees.
That's why.