Privacy

I presume the idea is that by separating the encryption step from the messaging app, people can be more confident that government agencies are unable to monitor their conversations. You know, by taking secret screenshots of the device or whatever.

But it seems to me that you could easily do this yourself using the gpg command line tool like so:

gpg --encrypt --armor --recipient <PUBLIC KEY ID>  

This uses the stored public key identified by --recipient to encrypt whatever you type and produce the result in ASCII text form, which can then be sent over whatever messenger you wish.

I'd prefer to do it this way than trust some unknown 3rd party app.

You could even pipe the output into a tool such as qrencode to send your encrypted message as a QR code, depending on message length. This would allow you to keep the machine doing the encryption fully offline.