When you write about things you know, you tend to write with more authority. This gives your readers confidence that you are a valuable source, especially if you add your own experiences.
When you write about something you don't know, something you want to learn, it comes across that way to your readers. They get the sense that you don't really have a clue about what you are writing.
So, I write about things I know. Sometimes you have a partial knowledge and in order to write about it you need additional research, that is okay. But if you are not familiar with a subject, it's best to learn about before writing it, rather than writing it while you learn.
An example is this past Halloween, I was encouraged to write about how my country celebrates it. I was even given a topic, but not knowing anything or ever experiencing the holiday I refrained from writing about it lest I do a bad job.