
In my field, the turn around time for an article to go from submission to publication in a peer reviewed journal is something like 8 months if all of the reviewers love your paper and think no corrections are required (which never happens).
arXiv lets us read the paper while we wait for it to go through the peer review process.
Also, arXiv allows the author to be able to prove that he came up with this result by a date everyone trusts. If some insensitive clod comes along later and claims they derived this result first, we can all point to arXiv and state the date that this author announced his result.
Living on Earth may be expensive, but it includes an annual free trip around the Sun.