Rol makes a good point about the failings of Hyrule Field. I don't think it is fair to say the field is an excuse for Epona (or vice versa) - the two are obviously complementary, but one's purpose is not limited to that of the other.
Still, what is the purpose of Hyrule Field? I think it's primary role is simply to lend a sense of scale to the game world (for the record, the field is hardly non-linear). A simple matter, but essential to the experience of the game. World-building is, I think, one of the primary strengths of video game fictions. Ocarina of Time's epic journey (in the classical sense) consists of traversing the world, meeting people, and conquering dungeons. It is no coincidence that the world features many races (eight in total), many locations, and many mysteries, and that these components are directly linked to one another. The concept of the world telling (much of) the story dates back to the original Legend of Zelda (though it was obviously a lot more primitive then).
Of course, an unfortunate consequence of this sense of scale is that it needs to take time to traverse the world - and so far, Nintendo has largely failed to provide meaningful content to fill out the vast expanses of land (or sea). Will Skyward Sword be different?
In any case, Aonuma recently touched upon this (and a related matter) when he talked about how Ocarina of Time felt like it was a grand story in spite of the relatively thin script. The player's relation to the world is key to this phenomenon, and it is good to know that he has a sense of it.
The other thing Aonuma's insight alludes to is this: a game is not a narrative. Modern games typically have narrative structures, yes, but the game itself is a generator of narratives (it is, in this sense, something of a meta-narrative). For example, what about a fight? The game sets the stage for the scene to take place, but the player instantiates the sequence of events leading to the outcome (narrative). What's important to realize is that any such scene will utilize a narrative form, becoming part of the story told by the game. Many people fail to appreciate this, I fear.
