[paisible]

There are a subset of applications that imo could rely exclusively on email as the platform.

A few months ago I built such an app, a small side-project called betabrokers(email [e-mail] with subject:about to signup and learn more). The simplicity of the signup process for this type of app (simply sending an email) is a huge benefit.

Email-based apps especially make sense in a mobile context, which is somewhat ironic because the transactional nature of applications built on them would suggest that they are slower, therefore less likely to be used. In our case however, users keep coming back to it, probably because the email client is the single most used app on their phone, and receiving replies to your actions via email becomes addictive.