The Information's Jessica Lessin reports:

"The purpose of a deal, which is still in the works, is to support the development of SpaceX satellites that could beam low-cost Internet around the globe to billions who don’t have it."

A source close to the alleged deal says Google "has agreed to value SpaceX north of $10 billion."

So this is about the side goal of SpaceX (get Internet to everyone) instead of the main goal (Mars).

From Eric Jhonsa, Seeking Alpha:

• The reported goal of the investment would be to "support the development of SpaceX satellites that could beam low-cost Internet around the globe to billions who don’t have it." Google was reported last June to be looking to spend anywhere from ~$1B to $3B+ on a satellite constellation providing Web access, and soon afterwards bought satellite imagery/analytics firm Skybox Imaging.
• Musk and SpaceX, meanwhile, are just days removed from unveiling plans for  an Internet venture involving hundreds of satellites traveling at a much lower orbit than traditional satellites (thus reducing latency). Musk: "The speed of light is 40 percent faster in the vacuum of space than it is for fiber. The long-term potential is to be the primary means of long-distance Internet traffic and to serve people in sparsely populated areas."

Haha so now Elon Musk is building his own Space Internet! Brilliant.

And useful when you think about everything he does as being conducive to going to Mars. 

Larry Page's comments about Elon Musk foreshadowed this investment:

http://pandawhale.com/post/39646/google-ceo-larry-page-elon-musk-would-get-my-inheritance-over-charity