On NPRs Talk of the Nation (December 19, 2007) Liz explained that this is a frequently asked question. Here's her response --
"A little something called a book advance, which is not available to every traveler I'm well aware, but it's my fourth book. So before I went on the journey, I presented it to - I mean, I wouldn't have been able to go on the journey, especially after a really expensive divorce, so I presented it to the publishers and said, I'd like to write a book about this and, truly, I did want to write a book about it.
You know, as Joan Didion said, writing is the way I found out how I feel about something. I would not necessarily have wanted to go on that journey if I couldn't have used, you know, my craft of writing to sort of figure my way through it. I think it also - you know, knowing that you have to write about something makes you show up for the experience a little bit more. It kept me from being lazy. I knew that at the end, I had to distill all this and understand it, so it forced me every day to pay attention and to literarily take notes."