I recently finished writing a chapter on travel writing for the Cambridge Companion to Creative Writing (due for release next year). The chapter argues that much of the pleasure of travel writing lies in its combinations of forms: of all types of writing, it relies most on incorporating at least some aspects of exposition, description, narration, analysis and argument, and humour. It is a genre of mixes.
For example, someone like Bill Bryson uses a mix of narration and exposition to create both a sense of lived experience and a sense of place - for Bryson, the link is formed by tone, or his attitude (witty, loving, silly) to the subject matter. Robert Byron has less of the Bryson-type personal narration, but greater amounts of description - the unity of Byron's The Road to Oxiana lies in his style, which varies considerably but which is always sharp and very fine. But the liberating thing about travel writing for the writer is that it allows them great freedom in how they manage the combination of forms.
What I didn't address in the chapter was the broader question of why we write about our travels in the first place. I doubt there is an answer for everyone. One way of re-phrasing this question is to ask, what does writing give us that travelling, on its own, does not? And the answer to that must vary greatly.
For me, writing is almost always multi-directional and, at its best, a cheating of time. Unlike real-time travel, which is at some level always hostage to a sense of destination and the limits of time, travel writing is completely open-ended and timeless. The ultimate destination of a real destination could lie in the past, in a dream that you had as a child, in an ambition, or in a moment that you've never fully resolved. Or the trip may in fact be taking you ahead, towards a book that you're yet to read, but which when you read it will make sense of your journey.
Because it exists in language and not in budgets or train timetables, travel writing is unbounded in a way that actual travel never will be. In the case of travel, which is so often constrained by practical matters, language enables a second, endless journey to be anchored onto the first.