Here’s the definition of LSI use from SEOBook.com (taken from http://www.seobook.com/lsi/lsa_definition.htm):

Latent semantic indexing adds an important step to the document indexing process. In addition to recording which keywords a document contains, the method examines the document collection as a whole, to see which other documents contain some of those same words. LSI considers documents that have many words in common to be semantically close, and ones with few words in common to be semantically distant. This simple method correlates surprisingly well with how a human being, looking at content, might classify a document collection. Although the LSI algorithm doesn’t understand anything about what the words mean, the patterns it notices can make it seem astonishingly intelligent.

So how does this effect you and your Plain Old Semantic HTML? Well, for starters it will give you a better grasp of your sites overall theme, and how closely matched the context is in correlation to each other. Since Google has already been using this type of technology in it’s algorithms, and it looks like the other SE’s are following suit, I urge you to really analyze your keywords within content for their semantical relationships and values.