Users tend to have one of two behaviors when interacting with a list of news stories or email: after they read an item, they either read the next in sequence or they return to the main list to select another story. As users become more adept at managing a particular information source, their strategy will shift between one of these and the other. For example, when reading my inbox I typically pick things that will be fast to deal with first, clearing them out. On the other hand, when reading my mailing lists or news folders, I simply read them in sequence. Some RSS feeds I simply read in sequence; with others I skim the headlines.
The design of most email desktop software supports both methods, with the list of email up top and the actual message below. The desktop-focused web mail clients support both to an extent, with "Previous message" or "Next older message" or some such controls on the message itself.
On the mobile it really isn't possible to simultaneously display the list and the message due to small size, so the list and items need to be separated. This doesn't change the user's desired behavior.
The list navigation pattern supports both behaviors. The mobile Gmail applications, both web and local, don't (at least not the four versions I tried). I miss them.