[in my opinion] Don't write to be read. Write for you, write because you enjoy the process and the exposition helps you process and crystallise a concept. Write because you are interested and engaged.
You can definitely do it in a private journal and would likely experience many of the same effects. But putting it online gives other people a chance to learn from it.
Sometimes because publishing helps people stick to doing it. Sometimes because publishing forces someone to a higher standard of care. Sometimes because the mere possibility of feedback or a connection is worth it. :)