Go already supports many features that functional programmers might want: closures, first-class functions, errors as values, etc. The main feature (and in fact only feature for now) that Fo adds is type polymorphism via generics. Generics encourage functional programming techniques by making it possible to write flexible higher-order functions and type-agnostic data structures.
People have been asking for Generics in Go since the beginning of time. (2009)
At this time, Fo should be thought of primarily as an experiment or proof of concept. It shows what Go looks like and feels like with some new language features and allows us to explore how those features interact and what you can build with them.
Perhaps Fo’s author is hoping that enough traction/excitement around this project will convince the Go team to add Generics to the language.