While Chicago is indispensable for anyone working with words, the book can be a beast. (As a copy editor at The Stranger, I reference it regularly.) It's big, it's rife with intimidating words like predicate nominative, and until you wear a groove into the sections you'll flip to over and over again, it can take some digging to find a simple answer. So the web version is a dream. Not only is the content easily searchable and hyperlinked, you can also bookmark sections, take notes, and create your own style sheets with links back to various rules.
Chicago provides simple solutions to recurring queries. The last edition was too wishy-washy for some, providing multiple options and leaving room for flexibility—commenters writing to Chicago's popular online Q&A complained that they just wanted to be topped, told what do in a definitive way from an authority in the field. So the 16th edition simplified some things.
If you like grammar, or if you'd just like to read an accessible explanation for why grammar is important, you have to read this. It's a smart, funny piece by a smart, funny professional grammar nerd.