Why do we need private messages on sites?

Hello! Why do we need private messages on sites, if each of us already have our own good mail? Why not just make an email gateway that would give out pseudo-email addresses and redirect letters from these addresses to real ones? Let me explain with an example: there is, for example, a user of habr, smallandpurple, who will send a letter to torpe@habr.ru from his post, and habr, in turn, will redirect this letter to my address big-and-greep@gmail.com, changing the sender to smallandpurple@habr.ru

All this can be done with the help of Google and its Google App, for example. Or without it.

A simple idea?