I was explaining what monads are. You have read some of the weird misconceptions about haskell and monads and are now repeating them.
I used monads only as an example (in brackets) but it's hardly the only form where purity creates clear divisions in the codebase. So, once again, the specific functionality of monads has nothing to do with it. I'm speaking about the coding style that Haskell promotes making detecting bugs easier during code reviews.
Having done security reviews of code I would have loved to have that type of distinction exist in a codebase compared to the usual OOP mess (with PHP or Ruby for example, which is typically the only paid work available in infosec) where state/user input is leaking all over the place. Compared to clearly defined paths containing IO - which are separated from pure functions which don't have unexpected side effects and require far less scrutiny.
I'm answering my own question here but I was hoping someone who has more experience than me at conducting security-centric code reviews would chime in to provide their perspective.