Too much of a good thing? On the other hand, the high-density information typical of face-to-face exchanges can be too much of a good thing; many clients would prefer to limit information availability. One benefit of less information is that it can help to focus an exchange, making it more efficient. There is much less socializing in email work, we tend to get right to the point. It also helps to minimize or eliminate extraneous variables: factors such as age, appearance, or foot odor, for example, are irrelevant in an email exchange. 
Discussing sensitive issues while facing another human being can be rather unsettling... or next to impossible. Many of my email clients have bluntly stated that they would never endure the anticipated discomfort associated with working on their issues face-to-face. E-therapy virtually eliminates, or at least reduces, the perceived risk of self-disclosure. Indeed, online behavior has repeatedly demonstrated a disinhibition effect: netizens are more willing to speak their mind online. E-therapy, then, makes it easier to talk about the things clients need to talk about.