At least in economics it can easily be 1-5 years until you go from draft to journal. In the meantime, you want a way for others to easily cite your paper, to make different revisions available, for you to post it in a way that's stable (people's websites change all the time, etc.)

Also, because most folks don't want to deal with paywalls, it's standard practice to put the last version of your draft before conditional acceptance on an online repository. It used to be SSRN for econ/finance, but they sold out to Elsevier, so now arxiv is increasingly being used.