I think Google just have done that for two reasons
0) Google intended to gather still more and more data on the Internet usage habits of the world's users.
1) Google wants to thwart ISPs' common practice of intercepting mistyped URLs with a page of ads.
the IPs of Google DNS servers are 188.8.131.52 & 184.108.40.206
in simple words if you have problems with DNS and wanted to use alternate DNS IPs of your ISP, you can use the above two IPs
official announcement on Google Blog could be found h3r3
Google Public DNS page could be found h3r3