SEO plugins have been around for many years -- with most of the website traffic coming from search engines, it made sense to optimize content for search engines (thus the term "Search Engine Optimization"). But the market has been changing over the last few years, and now many websites see 50% or more of their traffic coming from social websites -- and when you share content on social websites, they need different things than a search engine does.
SEO plugins have tried to offer some social features, but they can only offer the basics -- letting the social website know about the title, a short description, and maybe a default image.
But what about your images from NextGEN Gallery? SEO plugins don’t resolve shortcodes to retrieve images. How about videos? With preview images? SEO plugins don’t use Youtube / Vimeo / Wistia APIs to fetch that information. How about WooCommerce / WP e-Commerce / MarketPress products? SEOs don’t offer information on prices, variations, stock levels, categories, etc. How about bbPress forum / topics / profiles?
All of these, and much, much more, is covered by the NGFB plugin -- it covers the other half that SEO plugins don’t. ;-)