I started posting books a year ago when there were some I wanted to read that were missing here and I've continued that way since then. I've also added a few that people had requested specifically and I've posted a few that I simply thought would round out our collection a bit.
As to proof-reading the text before posting, that is certainly not a requirement, in my opinion. If it was, I would not be posting books here or very few. Most of us convert books because we want to read them and proof-reading the text first is a horrible punishment to have to endure before you've had the pleasure of reading it first. So I convert and post the books and then, as I'm reading or rereading them, I keep notes that I intend to use to make the corrections later. That seems only sensible to me.