Haskell has to have an escape hatch in order to work, though fortunately it is hidden away.
Every program has effects. A program that does not have any effects (IO) would not be useful, as you can't get anything into or out of it.
In FP we manage those effects, in order to help ensure correctness, with the additional benefit that good effect management makes the program easier to comprehend (nee reason about).
Contrast a procedural program with effects littered throughout the codebase, with a program wherein effects are pushed to the edges. In the latter, you and your team know exactly where all of the effects occur. Everything else is pure code: easier to test, easier to comprehend, easier to compose.
Category theory is not required for good effect management. It just so happens that monads like IO fit the problem space nicely; although the same could be achieved with a lazily evaluated definition of computation (i.e. a function).
Every program has effects. A program that does not have any effects (IO) would not be useful, as you can't get anything into or out of it. In FP we manage those effects, in order to help ensure correctness, with the additional benefit that good effect management makes the program easier to comprehend (nee reason about).
Contrast a procedural program with effects littered throughout the codebase, with a program wherein effects are pushed to the edges. In the latter, you and your team know exactly where all of the effects occur. Everything else is pure code: easier to test, easier to comprehend, easier to compose.
Category theory is not required for good effect management. It just so happens that monads like IO fit the problem space nicely; although the same could be achieved with a lazily evaluated definition of computation (i.e. a function).