Because it forces you to be more attentive to ideas around you.
When you share what you write with others you can’t just share what comes to your mind.
You think through and make what you write clear enough for others to understand.
Through this, you get clarity of thoughts.
All this helps even when you are not writing.
Most importantly when going through, and documenting, and dissecting news ideas, you learn something new.
You grow fast as a result.
Which is an added advantage, in addition to building a following through your writing because it helps those who read what you write.
Happy writing in public.