publishing - Why upload to academic preprint sites like arXiv? - Academia Stack Exchange

Use the link to access responses to the following question posted on Academia Stack Exchange: "The recent question about the legality of uploading to arXiv made me realize that I don't have any understanding of why one would upload to a preprint site. (I don't work in a field where they seem to be common.) Why would one be concerned about getting a manuscript on arXiv, if it's already under review with a journal? Is it because of the long publishing timeline of journals? (That could explain why computer science — which publishes much more in conferences — doesn't seem to do much with them.) ..."