It does exactly what it says...
I write a ton of articles and one of the things that helps with SEO is to provide both outbound links as resources to the reader, as well as internal links to connect the website and show how each page is relevant to the next, etc.
If people click on the internal links, that's great for my website. When they leave my site - eh... not necessarily bad, but it'd be better if I can keep them on my site a little while longer.
I think that these link previous could potentially help with that - we'll see if it has an impact in the long term or not.
Either way, definitely a worthwhile purchase. I bought it, activated it, and added the code all in a matter of 5 minutes and then previewed it on my site. Everything worked like a charm.
There is a slight delay in the preview load time, so hopefully they can improve on that, but hey - for the price, it's a great buy either way =)