> b) why would a company as big as Google care about what a single user does?

Nice try, Google secret agent :P

No, seriously, Google is interested in what you browse and already tracks it (anonymously, I hope) to target ads.

So it all leads to your first point. There's really no difference in practice. But in potential it's true that a device with a cellular network connection can provide more interesting location infos than a desktop tied to your desk.