I think that's over-simplifying it.
Posts can have meaning and purpose even if there is no audience, it's all down to the intent of the author. If they do happen to land "with someone in some useful way" then that's a bonus.
There is a difference between worrying about whether you are being read and worrying about what you are saying.
There is a difference between idly wasting words and posting when you "simply can't know what, if anything, happens to them after they've been shared."
There must be a underlying reason why someone writes a blog rather than senselessly throwing the literary equivalent of faeces at social networks.
If something, whatever that may be, prompts someone to show up every day, even with an uncertain audience, then that's the passion, that's what drives the message.