There are actually two ways to interpret that question.
Who to write for as in a more literal, who-is-your-audience type way. As in, who is going to read said work. The intended audience. Like if you are writing a post about start-ups, then your audience is someone in that business or interested in being in that business.
The other way is the more metaphorical sense. Who are you writing for. In the greater way. Like why are you writing in the first place. Your internal motivations for taking your thoughts and putting them onto a page.
In an ideal world, with any profession, your primal motivation should be for yourself. Now, why you want to do it or why you enjoy it, that can vary. But you should do something because you want to.
Yea, yea, the world we live in isn't ideal. But there are choices and options available. Life isn't wonderful and you can't get everything you want. You can, however, get SOME of what you want, and do some of the things you want to do. Carve out a sector of your life, that is well, your own.
If you want to write something, write it. If you want to make something, make it. For yourself and no one else. Out of love, not necessity or obligation.
The work will show that.