There's a blog post currently making the rounds, about a developer's experience of using F# for anything serious for the first time: http://www.knowing.net/index.php/2014/02/13/notes-on-my-first-real-f-program/
Among other things, it touches the subject of parentheses. This is a subject that seems to be widely misunderstood, and the blog post exhibits one such misunderstanding by calling parentheses in F# optional.
I will try to bring some sanity to the table, but am fully aware that I don't know a fraction of what I would need to know in order to call myself an expert in F#. Comments and corrections are very welcome (bergius on Twitter).
TL;DR: It's all about precedence and associativity. It has nothing to do with differences between methods and functions. It is not the parentheses that are optional, but the spaces between parentheses and other things.