It's just a plugin that puts an opt-in box on your blog, right? I don't know what you want to know about it, but lots of people insist that you build an email opt-in list so you have a list of targeted customers to market to in the future.
For example there's an ebook called Double Your Dating, and they pay affiliates $40 for every copy of the book they can sell, even though the book only costs $20 for the customer. So DYD is willing to lose $20 on every initial sale. That's because every sale requires a newsletter opt-in, and it helps them build such a huge opt-in list that they make big money on backend sales (for example a monthly subscription interview series, dvd sets, etc.). Most people don't go so far as to lose money up front, but that shows you the power of a list.