It's sheer and utter and complete usefulness.
Sure, at first glance it's just a place for nerdy knitters to get together and act the part, but it's really a great tool for learning. The site admins have really made the site very intuitive and user friendly.
Since it's sort of a Bulletin Board, you can post back and forth on all sorts of topics. Who knew there were so many knitting Foo Fighters fans in the world? I so wish there were more sites set up this way- a sewing one in particular, but the possibilities really are endless.
but since it's also sort of like wiki, there's a lot of great actual info available. If you have a question, you can search the threads, or the techniques, or the patterns. This is key because there's no trawling through pages and pages of info you can't really use because the search hit on the word "the". There's a directory of useful places to shop. so say you need to find a particular yarn or needle, search the yarns or search the shops. There's a directory of people, so you can see where people are from, and what they knitted- you can ask them how they liked the project, the yarn et cetera.
my favorite thing about ravelry, is everything. I've really improved my knitting knowledge and skills in the few short months i've been a member. Rock On Casey, Jess; and Bob too!