The idea is to utilize the situation of change; when users have finished reading an article and are looking for something new to do. This exact moment is when to intervene and catch users’ attention, as they are changing state from reading to browsing. They are going from a situation of flow (reading) to having to make an active choice: they have to make up their mind of whether to continue on the same site or continue elsewhere.
Users are prone to continue online activities elsewhere in this situation of change, why it is a critical moment to act for the website owner when it comes to increasing pages per visit, lowering bounce rates, and increasing a dozen of other statistics.
For the website owner, the alternative to making a pop-up and attracting attention at one particular point in time is to try attract attention all the time. So even though a pop-up like this can be seen as obtrusive, this form is perhaps the least obtrusive of its kind. Consider and design for your own pop-up to be a helper rather than a moocher of attention.
Check out more examples of continuation prompts