How does a
+1 on a post (from Google Plus) that carries a link help for SEO? Will it help the PageRank in Google? If yes, how?
All in all - One share (+1) probably has a very minimal impact. However, social links are great in the sense that they can have a knock-on effect. i.e. Person visits your website via a link on Facebook and likes the content so much that they share it on their blog (not to mention the fact that nofollow links are a vital part of a healthy backlink portfolio).
PageRank alone != SEO.
Google attempt to rank by relevance, and PageRank is a single signal of that.
Google rarely reveal all the signals they use to rank a site, but the fact they own all Google Plus' data, would mean I would find it would be odd for them not to mine it to help improve their search results.
A popular site will be +1d, retweeted and liked - which are all good signals that a site is popular though, like links, open to abuse from spammers.
It is also good free advertising. Who cares how people find your site (whether it be via a Google Plus profile or via search), as long as the right people do so.