Your website could come up high and receive qualified traffic

As well as getting extra visitors from the +1 clicks, it is very likely Google will be using the data from those clicks as a strong factor in deciding how high it ranks a website against someone's personal profile at Google. This could mean if you are getting a number of +1 clicks on a page and someone who is logged into Google is doing a related search, you may come in on a very high position for them on Google and so get a highly qualified visitor.