Or, instead, you get a much larger audience reviewing your article, instead of the 2 to 4 reviewers involved in the paper publishing process :-)
You can get comments from critical and interested readers, you upload a new version of the paper to arXiv, and repeat the process until the article is ready for submission/publishing.
In other words: I think that an article on arXiv (on average and as a whole, individual articles may be exceptional) is subject to much wider and more extensive scrutiny and verification than the average article during the paper publication process.