[xupybd]

To me the big difference between OO and FP is complexity management.

In OO complexity is hidden. So you don't have to deal with the complexity of the internal state of an object while using the object. It's a divide and conquer approach.

In FP complexity is constrained. Pure functions and immutable data make it easier to reason about the code. This allows you to see all the workings and not get overwhelmed.