Comment by tikhonj

Functional programming à la Haskell has always been about making effects controllable, explicit first-class citizens of the language. A language entirely without effects would only be useful for calculation.

The talk about "purity" and "removing side effects" has always been about shock value—sometimes as an intentional marketing technique, but most often because it's just so much easier to explain. "It's just like 'normal' programming but you can't mutate variables" is pithy and memorable; "it's a language where effects are explicitly added on top of the core and are managed separately" isn't.