Who says you need to have a pop-up on your site all the time ? Display rules are very important to create display scenarios and make sure the right message appears at the right time to the right visitor. As an example, Zalando is probably testing various pop-up templates. This one displays apparently only when coming from adwords to the zalando’s  homepage, with a 10€ welcome offer. It is a smart way to increase ROI on adwords brand campaigns.