Are You Logged In?: "Before creating the Google accounts, Google used to rely on cookies to save preferences. The new accounts are a way to save personal stuff and all Google services have something to save. But not everyone visits Froogle Wishlist, or Search History, not many people have heard of Google Spreadsheets or Picasa Web. But everyone has heard of Gmail and a lot of people use it, although it's in beta and it still requires invitation. I think Gmail was created to make people have a Google Account and remain logged in.
(Via Google Operating System.)