When you search a web site from outside using a search engine and having the addendum site:mysite.com you have a good chance to find what you want. At least you get close to what you want to find and can either improve your search string of keywords or navigate to related content. The internal search engine work pretty much in the same way, even if the quality of the search result may vary from those from an external search engine.
There are sites that do have embodied agents where you can ask real questions (see image: question on the left, the answer I wanted highlighted on the right) instead of a keyword search. In this way the user have multiple choices of accessing content (regular navigation, external or internal search and the embodied agent). But from what I know, the embodied agent isn't very common on web sites. It might be that it is a tradeoff between cost and gain. Too few users and high cost is often a factor, and there might be other usability improvements, with better effect, than the embodied agent for the same amount of spending. Or there might not be any gain at all having the agent, and that is why they are rare on web sites.
Still one wonders... Do embodied agents improve usability?
More to read on Wikipedia: