> Just do some reasoning about the side effects.

"Wat?"

Have you ever inherited a 1MLoC codebase that you yourself didn't write from scratch, or worked on a large project with at least one other person? How do you know what the side-effects are and where they happen when you aren't intimately familiar with the code you're interacting with? That's the whole point of making effects explicit and available for analysis in the type system.