There's a section in there where you have to list all the countries you've visited in the last ten years. That's quite a long list for me, and one that I don't tend to keep up to date. It can be hard to remember whether I was in Germany in 2006 or 2007, and delving through email for e-tickets just gets confusing.
Luckily the annual travel meme, with its list of countries visited in the last year, came to my rescue. I could do a quick search on my archive, pull out the relevant postings, and fill out the form.
(Now all I have to do is get to the US Embassy for 8 am tomorrow...)