1. A lot of people have written English much better than spoken. 2. International (and somewhere even intercity) calls are expensive. 3. You always have history. You can show it or see again after a while. You can copy-paste something. 4. It is convenient. It is much more interactive e-mail, but less interactive than phone call. You can do something else when talking via IM to somebody.
some jabber software supports reasonable security. i.e. well-written clients properly check the server's cert and to c2s encryption to the server. a few really good clients will do e2e crypto using gpg keys or whatever. the gang of us who maintain our racks in westin and ashburn use jabber. i think of it as just another path. when things go badly, which paths will work and which not is often different than one expects. randy