Imagine a friend from overseas is coming to visit for the first time. There's bound to be something in your locality that they just have to see during their stay...
... translate this to your campaign world. What are the real stand-out places that are so memorable that books are written and pictures painted about them, that people want to go and see? If you haven't got any there yet, this book will give you pointers to making the sort of features - natural or built by sentient hands - that would have guide-books written about them. If nothing else, this adds to the 'real' feeling of your alternate reality, even if they merit no more than passing mention and don't feature in your plots and adventures.
The approach is very logical. Wonders are, by their nature, unusual so there's a formula to figure out how many an average kingdom or other such division ought to have. Then they can be natural or artificial, and also categorised as minor, major or truly unique. There's only one Taj Mahal or Grand Canyon on this planet, after all, although many countries will proudly point out their must-see buildings or natural features.
This leads on to notes on various types of wonder, and even their construction and upkeep costs... indeed, notes on in character creation of wonders, should you want to have one being created at the time your characters happen by, or if they have a mind to leave their mark on the campaign world by setting one up.
It's not what most of us think about when world-building, but now I've been prodded into thinking about it, I certainly shall next time I'm dreaming up a place... and not only for Pathfinder either!