>That sounds like an interesting article, would you send me a copy later on

This one makes sense to me since it roughly gauges the non-verbal aspects of interpersonal communication. If the interviewee does send a copy, it's a signal that he's dependable in some sense but perhaps more importantly if he goes a bit beyond and sends a few other articles on the topic, it shows passion for the topic and a desire to communicate that knowledge to others.

Presumably to see how attentive to details the candidates are?

I have worked with some people would would promise things and the immediately forget that they did so. This was extremely irritating. There are also people who never bother with conversation context when sending emails.

Asking for a trivial thing which needs to be done after interview shows the response to both of those questions. Will candidate forget this completely? Will they send an email with no subject or body and just an attachment alone?

I use the verbal ones for interviews where I haven't had time to prepare, or they are not there in person.

Really, listening to a request and following up is just one of those key things. If I had them come in and do a longer interview which I had prepared for, I still find it useful!