Exactly. They have nothing to say, but yet feel compelled to say something no matter how irrelevant or off topic it is.
Weird. This is like having a conversation with a hard of hearing person who thinks you said something else. But it's in writing. 
You hit the nail on the head with the hearing-impaired analogy. I know because I am hard of hearing. Rather than asking people to repeat what they said, I respond with an affirmative nob of comment. It is safer than saying no. Pretty sure I come off to folks who don't know me as being even more of an idiot than I am.

-Got new state-of-the-art hearing aids a couple of weeks ago which work like a charm. My old hearing aids were useless. It is interesting to hear almost all of what someone says.
Unfortunately, now I also hear my great-granddaughter who is in the terrible twos throw screaming tantrums several times a day. When it becomes overbearing I take out the hearing aids for some peace and quiet.