Once a WordPress powered site starts getting quite a bit of content, the default built-in search becomes fairly useless. It just isn't very smart. If you wrote a comprehensive article about He-Man, but since have written five other articles that just mentioned He-Man in passing, a search for "He-Man" will turn up your comprehensive article sixth. There have been various tweaks and plugins and whatnot to try and make this better. But why not leverage the best search engine ever written instead?