This enables sender and receiver to interact with their message autonomously and without distracting cross-talk. Email enables users to schedule interrupts. In one sense, email distractions are on-demand. Of course this may only delay the inevitable. More on that, below.
Email is both paperless and archivable by default.
Email can be very efficient and, in the context of desktop automation, is also very convenient. The efficiency derives in part from the fact that both sides of email communication may be completed in isolation - one doesn't listen to the other party. Email appears to be free, or at least cheap to the user. The appearance is illusory, of course, as free lunches are as hard to find in cyberspace as anywhere else. In either case, the charge is hidden.