The creator of Eberron decided to sit down and get out of his head all of the bits he'd never really gotten to explore in another book.
This was an excellent idea and worth absolute every penny. You get really solid setting information about Eberron's planes, religions, and several under-represented chunks of the setting, such as Droaam, the Dhakaani, and undersea stuff.
This expands out for both players and DMs options that you can choose to use or not, but adds a grounding and depth to the setting.
It's not a "must have" to run a game in Eberron - you could get by without using just the core setting book if you needed - but this is so jam packed full of ideas and concepts to play around with that you really, really should get it.
Little details fill every page, and there's obscure sentences you can hang entire character concepts off.
This book is a joy.
[5 of 5 Stars!]