Why does Google sponsor free open source software?

Google sponsors open source software through the Google Summer of Code.

http://en.wikipedia.org/wiki/Google_Summer_of_Code

Some projects that benefit include:

• Open Street Map
• Organic Maps
• VLC
• LibreOffice
• Gimp
• Debian

Why do they do that? Google is one of the world's largest corporations.

Isn't this... very suspicious.... ? Is their goal simply to fuck with Adobe and Microsoft executives?