An excerpt from http://www.sitepoint.com/wordpress-popup-plugins/, by @jmcmillen89
Popups are essentially the Justin Bieber of your website’s design. Everybody says they hate them, but somehow, they get ridiculously good results.
Popups work… really, really, really well. I’ve yet to see a case study where optin popups decreased the site’s conversion rate, and I’ve seen countless case studies where popups catapulted CRO by as high as 300%.
If you’re not convinced, don’t take my word for it. Simply use one of these top popup plugins for WordPress. They are quick, easy, and several are completely free.