Just think of the greatest adventurers who ever lived and the greatest journeys ever undertaken: the Jews, Marco Polo, Christopher Columbus and Charles Darwin come to mind. All of them had compelling reasons for setting off on dangerous journeys into the unknown. What they found (in their cases the Promised Land, China, America and evolution respectively) soldered them into history and made them famous, but also opened the world to travel as never before.
Travel writing ever since has echoed the odysseys of these great people. Writers still feel it incumbent on them to have some higher purpose to their journeys beyond mere self-indulgence or curiosity. On the rare occasions when travel writers break this rule they tend to fall ill or become irredeemably cranky when they sit down to put their experiences on paper.
The range of reasons travel writers dream up to focus their journeys range from the absurd to the sublime. Take that outstanding wordsmith Bill Bryson. This man literally thought up journeys he could take, to create fodder for his witty irony and superb humorous descriptions. A walk along the Appalachian Trail with an old school friend (do you remember...