If you aren't interested in reaching new people, why have a public blog?

I’m elevating a comment I made in a prior thread. We’re chatting about the Nature Network exercise in self-reflection about the insularity of their blog community. I made two points.
True that there is a population for any blog that should theoretically be in the audience but that departs because of the “tone”. Allegedly I, for example, lost some readers because I extend latitude to certain antediluvian commenters. It should be a consideration.
What should also be a consideration is who is being excluded because they simply do not know you exist- individual blog, blog consortium or even the whole science blogosphere.
It was a good critique, but there are always going to be tradeoffs.I happen to think that one of the good features here at Sb is that we start with a very open approach from which individuals bloggers can tighten up if desired. This can be incredibly fine grained. I’ve had a commenter or three that others of my readers object to and want to know why I don’t ban them. Other Sb blogs that I share readership with may have done so. To the extent that we overlap in blog-interest, readers can find the content without being exposed to the ‘clownery of some of my commenters. Perhaps I lose a few voices, and I regret that, but I have to draw my own lines in making what I think of as my blog’s tone what it is.
There are other types of commenters who pervade other Sb blogs that simply don’t come around here and if they did express that type of behavior here would be moderated or banned. Yet I don’t think Sb is the weaker for the Pharyngaloids, ERV’s selfconsciously outre fanbois, Laden’s “the real” neandertals or even Ed Brayton’s libertardian halfthinkers. I think we are the stronger for it- as a collective blog enterprise.
There was a comment at that NN thread (see problem? I have no idea which blog it is on, see Munger’s comment about individualizing the blogs) about writing for her own peeps and not giving a hang about traffic. Why have a public blog? There are more private social media and fora. More generally, why have the NN blogs visible to anyone other than those who register and login?
The very fact one engages in *public* blogging says that one is interested in reaching new people. Period. After that we are merely discussing whether you are doing a good job meeting performance goals…
Right? Why are you putting stuff out there for the entire Internet to see if you don’t mean to reach people who might be interested in what you have to say, but you have no other way to reach them. In a word, perfect strangers. There are a plethora of controlled-access technologies that would serve the same purpose if all you were after was a private circle of friends.