Whether on the desktop, in the browser, or the mobile device - The Giants of computing technology want to control the way you interact with the digital word.
The take here on the Grid is that controlling the user experience is central to any company making software or interactive applications.
Google's revenue comes from advertising, but they are fundamentally a (web-based) software, or user experience company. Apple's revenue comes from selling hardware and devices, but they are fundamentally a software, or user experience company (the user experience company, many would argue). Even Facebook, Myspace, Digg etc are earning revenue from advertising, but if you stop and think about it, are fundamentally software, or more generally, user experience companies. (made valuable by virtue of facilitating electronic interaction for, and being a destination of multiple users)
Big bad old Microsoft, OTOH, still makes it's money selling software, but has to do web stuff & sell hardware in addition to that just to support and defend the legacy revenue stream.
The problem for a company like Microsoft is that software (like file storage & management, apps, etc) is going to eventually move to a web-centric, subscription based model.
The answer is ALL OF THE ABOVE. Ultimately the web is out in the cloud and on our local machines. It doesn't matter what the form factor is (pc, notebook, netbook, phone) or where the code running it lives.
If Google can control the experience it reinforces their other properties and future ones they roll out. It also paves the way for subscription based services, like file storage.
It's a multifaceted approach to be sure, from all players. At the end of the day, they all want the same thing: To control us, er...the user experience...and we let them, because they usually give us such cool toys to play with :)