Open source software is usually only as good as the supporting documentation. Imagine unpacking your new IKEA furniture and finding no instructions. You can probably figure it out, and the more pieces of IKEA furniture you have built the more chance you'll have to being successful. But wouldn't it just be easier if there were instructions?
For me, the biggest challenge of writing good documentation is dispensing of the idea of assumed knowledge. I have a bad habit of assuming that if I know something, everyone else does too. It can feel as if I am over explaining things, and yet the documentation I like to read assumes near zero knowledge.
Too much information is better than not enough.