While the standard is obviously written with OOP in mind UML is just as well a usable tool for desribing e.g. Erlang actors (sequence diagram) or the functionality buried in lisp packages. It helps drawing boundaries and responsibilities which might be just as important in FP.

The bulk of code generation tools and diagram "helpers" might be pointless in FP, but when you sketch on a napkin boxes and arrows just do their work in a way that even OO people can understand what you are talking about.