Yet another way in which people could use arXiv is as a repository for material which otherwise cannot find a home in a journal. Sometimes in the course of working on a project I wind up with some material which did not make it into the published article -- or perhaps some notes which record my growing understanding of articles already published by others -- which look like they could be useful to the community as expository or supplemental information, but which in my opinion are not otherwise significant enough to warrant submitting to a journal. I have sometimes wondered whether it would be appropriate to post such material on the arXiv. (I have not done so yet.) For that matter, I wonder whether others have done this very thing.
One drawback of this use of the arXiv is that everyone knows that most arXiv articles have not been peer-reviewed at the time of first posting, so they must be taken with a grain of salt. If an article is never published in a journal, you must read the arXiv article with a more critical eye. But as I said, I do believe that there exist some notes that are perhaps worth sharing but not worth wasting the effort to peer review.