During my typical work week, I get called into many consultations where clients, or prospective clients, need help understanding what a URL rewriter is and how to effectively implement one to improve search engine rankings.
A lot of times, prospective clients come to us because they cannot find their site in the search engines. When we investigate, we find a complex URL string with 4 or 5 variables. When we let them know that it is probably because of that issue, they say "but the crawlers have been visiting my site and indexing it."
While this is true, it is important to distinguish indexing from ranking.