It also has a LaTeX like input language. Essentially I view this as a much more convenient way of distributing your preprints and reprints. This has obvious benefits for mathematical research. They are archived. Their articles are much more easily searched.
Journals are of course very valuable to the mathematical community.
Putting your papers on your web site should not replace publishing them. Copyright issues are discussed below. It is especially important that the reader not have to install anything except his browser and that it is possible to view the document on a wide variety of platforms. The author should not have to maintain a separate version of his manuscript for the net version. Loading should be reasonably fast.