so you're supposed to start off easy, right? go by the book, go by the campaign setting they give you. just stick to the six core races, and don't go overboard trying to invent stuff, right? well, I didn't exactly do that. I created a whole world from scratch, messed with the core qualities of numerous races, core races or otherwise, and on top of that, invented a pantheon and mythos which is completely incompatible with Golarion's. this is the first game I will ever GM. to be frank, I'm in trouble.
with that context, this book is a godsend, and I'm glad to have the freedom and ease of use this guide gives me. having a game world populated with multiple monstrous races (most of them completely reimagined), I needed to have a way to make sure the stats reflected the people. it breaks immersion to have a race with traits that quite clearly do not make sense for them. something that always bothered me with the core material is how race was treated: I found it restricted, stereotypical. clearly, the Pathfinder race system needs a little diversity, especially if your campaign isn't actually set in Golarion.
one clear example of the usefulness of this guide for worldbuilding and racial diversity is the Gnome trait "hatred". see, the rules state that the Gnomes have a deep-seated hatred of goblinoid and reptilian races, but in my campaign, Gnomes and Goblins hail from different corners of the universe, and logically, shouldn't even be aware of the other's existence. it simply wouldn't make sense for me to have a gnome character that's trained against a race they've never seen before in their life. thank god this guide has other plausible gnome traits that I can replace that problematic one with one with. not even to mention how the Ifrits, Oreads, Sylphs, and Undines had filled a gaping hole in my mythos. (though I was dissappointed to find that the Kobolds were still utter weaklings)
in another spur of greatness, I can already tell that my players, by now fairly intoxicated on the freedom I've given them, are going to love these new options. I can already see one of them deciding they want to go with one of the very comprehensive and imaginative archetypes, or choosing catfolk or kitsune instead of elf or goblin. the best part is, this book is so easy to figure out, so I am perfectly able to give them this freedom without puzzling over the rules for a month (like I embarrassingly did with the core rulebook).
trust me. if you're the kind of Game Master that doesn't like playing by the rules, and likes to do your own thing lore wise (like create a complete departure from the default setting), then this book is almost a necessity for you. for anyone else who likes the idea of monstrous PCs, you'll love this one.