Does really rich snippets (microdata) helps to increase the SERP position or it will be just for usability purpose?
if adding rich snippets to page helps, then how it will affects the SERP position?
As far as I am aware, they do not directly influence your SERP position. However, they do increase CTR (click-through-rate) on search results. A lot of webmasters work hard at getting themselves into the top five results for a specific keyword without ever giving much thought to CTR. This is a bit wasteful, in my opinion, considering our sole reason for trying to rank well is to gain more traffic.
There is also a belief among many SEOs that CTR does affect your SERP rankings, which would make sense, considering how useful a metric it is.