You manually enter information in order to override Wordpress defaults. It allows you to control the title, description and keywords of every post. For example, let's say you have a post named SEO All-in-One. And let's say your blog is called SEO Blog. The default title would be: SEO All-in-One | SEO Blog You can overrride this and make it something like: Search Engine Optimization | Wordpress SEO Plug-In | SEO Blog I personally like this plig-in.
What Brauner said is true. I will add this. This plug in is a secret weapon. I belong to a mentorship group where we use custom modified, souped up WP platforms with proprietary programming as well as free, and this is a free plugin we use. I have gotten double page 1 google rankings with it, the kind where 2 results are indented. How? I manually provided a different title, keywords and description. Google then picked up 2 relevant entries for the search keyword. This powerful tool lets you add specific keywords relevant to that particular post. This is distinctly different from tagging. Both need to be done. If you are not tagging, bookmarking and adding keywords, you may as well not be posting. This tool makes the SEO part simple. socialmarker.com makes the bookmarking simple.
So you got a couple site links out of it. That has nothing to do with the All In One SEO Pack, but instead of the quality and perceived authority of the content on your site. Still a cool thing to have though.
I myself have benefited tremendously from it. In February, when I first started using Google Analytics on my fairly new blog, I received 95 search engine visitors for the whold month. In March I got it up to 189; April 291; May to-date 643. My static site which sells an herbal drink in a narrow niche went from 77 search engine visitors and one sale in February to 136 May to-date and three sales. The blog has progressed much more, and I'm very happy with the SEO plug-in.