At its core, FX (feature expressions) allow writing a set of forms which provide different implementations depending on the 'mode' in which compilation is done.
While FX is great for convenience, the tradeoff is that it strongly encourages conflating the notion of 'how' (platform specific instructions) something is done with 'what' (logical operation) is being done.
FX lets you can now shoehorn all the different implementations (currently java & javascript, but probably CLR down the line) into a single function, walk away and pretend its a fully portable solution.