Email was once the user interface for all remote services. Bitnet had nodes that responded to commands and performed operations that sent the results in email. For instance, you could send an email to "TRICKLE@TREARN" on Bitnet with a subject line: "GET ftp.funet.fi /pub/something" and the Trickle service at TREARN node would download the file over Internet, split it in chunks and would send it to you over Bitnet, so you'd effectively have FTP capability on Bitnet just with email.

I had written a user database called "Hitbase" (a very primitive Facebook) on a Fidonet network that responded to Netmail messages to a given node and sent the responses to the requesting address. That was in the 90's before Internet was accessible from homes.