I do a lot of writing, but at least 50% of it is in my head. Like Borges’ protagonist in The Secret Miracle (though happily without the conclusion), I can write reams without putting pen to paper.
It’s quite handy to be able to do that, because it means I can “write” while I’m travelling or swimming or shopping. I don’t claim any great credit for this facility: it’s just something I have, not something I’ve worked at.

So, why not publish my mental efforts? Several reasons. 

First, if I write a diatribe, I have to weigh up whether actually publishing it will have a beneficial outcome. It may help me to get things off my chest, but that side of things is taken care of by the act of writing anyway. There’s no point in fuming in public unless something is likely to change as a result. Much better, in my opinion, to weigh the pros and cons carefully, and then take the most cost-effective course.

Secondly, I would have to spend time writing it out properly, and that is usually time I don’t really have. 

Thirdly, a much more prosaic reason: by the time I put pen to paper, as it were, in all likelihood I’d have forgotten what I actually “said”! 

Nevertheless, from a personal point of view, mental writing is good exercise, it’s enjoyable, and allows me to try out different beginnings and endings without going to the trouble of actually bashing it out. 

It’s self-directed, in that doing mental writing means, in effect, that you have an audience of only one: yourself.

Nevertheless, I’d highly recommend it.