In a pure functional language, the only thing you can do with a value is apply a function to it.
In other words, if you want to do anything interesting with a value of type a you need a function (for example) with type f :: a -> b and then apply it. If someone hands you (flip apply) a with type (a -> b) -> b, is that a suitable replacement for a?
And what would you call something with type (a -> b) -> b? Seeing as it appears to be a stand-in for an a, I'd be tempted to call it a proxy, or something from http://www.thesaurus.com/browse/proxy.