What's AJAX'ing search result pages do?
That means you don't need to reload and re-render an entirely new search result page when you type in a new search query. Only the parts of the page that changed need to be replaced (the results and ads). Re-rendering an entire new page is slower because the browser must look at the HTML, parse, render, and interpret JavaScript every time you load a new URL.
Why does this matter?
Money money moooooney. Site speed is a huge deal for search engines. Speeding up your results usually means more searches. Which also means more ad clicks, bing bing :).
I have a feeling this was in response to Bing being faster than Google when Bing launched. At least we know Google is on it's toes. I'm still a share holder.
Full disclosure: I worked at Google as a software engineer on AdSense from 2003-2007.
