I agree that it might not be like every other corporate venture arm. Although, if I had to speculate, I think Google is doing this because it needs to diversify its business outside of generating cash through advertising. By investing in ventures, Google should be able to supplement its income nicely, keep its stock price high, and not have in increase operating expenses too much should anything that they invest in takes off.
If I were a shareholder, I'd much rather have them pay me a dividend than enter the distracting world of venture capital. These types of moves inevitably risk turning the company into yet another bloated enterprise, even if they are successful in their investments.
Google has always had a significant amount of cash. They used to use that capital to do really interesting, strategically smart things, like buying dark fiber. This was similar to Apple's philosophy - you don't see Apple distracting themselves with some sort of "Apple Ventures" play; instead, they will take advantage of their cash position to do things like cornering the market for miniature hard disks, which leads to a direct benefit to the bottom line.
