I mean, there's different ways to describe this but it comes down to how you want to think about state. The more pure you get the more you can focus on operations or transformations, as functional programming is really about pipelines of transformation of code & data. By having such a constraint over state it allows one to potentially find it more easier to reason about whatever domain they are solving for. However in order to achieve that there is a certain set of hoops one has to mentally go through and maintain in order to think clearly about said problems. For some that is worthwhile for others not. I generally trend towards code that is functional regardless of the language.

Related concepts that are relevant at a high level is the comparison between declarative code and imperative. Code that is functional tends to be declarative and in a way "is what it seems to be." There is also a transitive property that allows the makeup of a thing to also be its own runnable representation which makes portability or idempotent things easier to achieve.

I dare say that original "OO" in terms of "message passing" is functional in nature, but object-oriented somehow became something it is not.

Erlang is probably the best representation of a functional language ideal in my own personal opinion when weighing in the ecosystem and capabilities of the language and tooling.