There are some wildly imaginative benefits of FP sold in words by some advocates, oftentimes imagining what could be in a runtime or compiler based on the maths of what’s promised by the practice. It’s easy to imagine that if an expensive function call always produces the same results it can be optimized to a single call, after which all further calls are free (a memory access). It’s easy to then overstate or imply that calling a pure function guarantees that kind of optimization even if it doesn’t.