LinkSpirit is internal link analyzer. Basically, your PR (pagerank) can be effected by dead links.
So instead of going through many many links and finding out if each one is dead, this checks the integrity of each link and then manages it from there. This way, it keeps your site clean and keeps your PR up to par.
It can also calculate if you will lose PR due to bad links, inventories which ones are bad ones, and what can or cannot be read due to your robots.txt file. It also checks syntax and meta tags, since those are other things that could interfere with the most optimal setting for highest PR.