What is this? Often on blogs or online media you will experience an overlay pop-up encouraging you to add your email to a mailing list, or register for the site to receive some other benefit. The overlay appears over the content, forcing you to either read the overlay or close it.
Why should I do this? The reason this is so prevalent is that it can be effective. The overlay method guarantees that your mailing list signup is seen, as opposed to having the form in a sidebar or footer where it can be easily glossed over.
Overlays can be a polarizing tool to use. On the one hand they can be effective for capturing user emails or introducing users to an offer that they may have otherwise not seen. On the other hand they can be disruptive to users whose primary intent was to view your content, not to sign up for your newsletter or register for your site. Like some other tactics this is one where you must consider the business benefit along with a degradation in general user- experience.