Is there any advantage of putting source tex file on arXiv?
Arxiv mandates uploading the source of any PDF file generated from Latex code. You will not be able to upload only the PDF version in this case (unless you succeed to game the system).

Why do the require the source? This saves quite a lot of space (50k-100k source vs. 500-900k PDF) and there are some additional reasons. As the other comments mention - the full explanations are in http://arxiv.org/help/faq/whytex