It prevents anyone from snooping on your web searches, unless they have the private key for Google's SSL certificate.
Whether anyone but Google has the private key is open to discussion. However, you can configure Firefox (and perhaps Chrome) to only use Diffie-Hellman key exchange. There's no known man-in-the-middle attack for that, even if you have the private key.
Unfortunately, you will find that you can't connect to some secure websites. If I turn off all cipher suites without Diffie-Hellman key exchange, I can't access my bank's website.
Maybe Google wants to corner the market on internet marketing via keyword?