It may because it is consider as votes or recommendations but I think it is not that high due to it can be abused.
First of all , its a mystery to many users how it works out.Google's +1 button is unlike Facebook's Like button , its totally different.A Google +1 can indirectly follow up to a better page rank.Yes a greater number of +1's increases a link's potential for a high CTR.[FONT=Arial, Helvetica, sans-serif] [/FONT]
Google's +1 button effects your seo and promotion activities , becoz mainly google is the one of the main search engine among others . Everyone going to search on google . google +1 is google's home product , so it is obvious google give more priority to google plus among other social media sites like facebook ,twitter ,linkedin ...
I think it helps only when you are logged in to your account and searching about your business then Google will show it on top.
...the SEO effects of a +1 are very indirect, and i still don’t get the true value of having a google +1 button on a website
It is like personally recommending your choices to your contacts; an extension of Word-of-Mouth promotion. A thing that gets +1ed more is actually a thing that is catchy and meaty. Such a piece gets more viewers and hence there is an increased probability of them being converted into customers. As far as optimization is concerned, Google has said that soon it will start counting +1s for assigning SERP rankings.