You mean why do they hire them?
To solve hilariously hard computer science problems and research new technologies.
There's also the persistant rumour of GoogleOS...
One of their new technologies is Searchmash. You can use it at the moment - its good, I believe it was designed to solve the main current problem with Google which is that it relies heavily on Pagerank, so it is very hard for new websites to get to the top of the results, even if they are the most relevant.
You also tend to get Wikipedia pages ranked highly even if they aren't the most relevant results...
It's really hard to design amazing search engines because (I can't remember the exact numbers) 95% of words on the internet are the same 500 words.
Pfft.. to be honest they probably just steal them to keep them away from Microsoft.
-= Out here in the fields, I fight for my meals =-