Symbolism, Imagery, Allegory
In our everyday speech, we constantly use metaphors of space, distance, and landscape to describe other things: how we're feeling, our progress in our work, or how we look at the world: "I seem to have hit a rough patch"; "The grass is always greener"; "I took a shortcut in the math problem"; "This argument's going nowhere."
In a way, it makes abstract things feel more concrete. This is precisely what Bunyan does with the landscape of Christian's journey.
Take The Slough of Despond, for instance. This place name is totally descriptive. Despond basically means depression. A slough is another word for a swamp. It's stagnant and murky, and you sink when you step in it (think of the Fire Swamp from The Princess Bride). So it seems like what Bunyan is saying is that feeling depressed is much like being stuck in the muddy waters of a swamp.
Of course, the landscape allegories that Bunyan gives aren't limited to their names. Rather it's in how the characters interact with the landscape that fully describes the idea Bunyan's representing. Think, for example, of the way Formalist and Hypocrisy choose to take easier routes around the Hill of Difficulty (P197).
In Bunyan's landscape, these two easier ways lead to the Forest of Danger and the Cliffs of Destruction… pretty neatly implying what happens to people who try to take shortcuts by only seeming religious or following formal ritual. Using features of landscape, Bunyan can really convey his sense of where certain actions, attitudes, and choices lead.