It's a subscription service. This means that when you write an article, all the people who subscribe to you will receive an email with your article in it. They need never go to your site.
In order to do that, some people charge a subscription fee. Substack doesn't actually pay you for your writing - your readers do. Substack takes a percentage.
I resisted for a year and a bit, but in the end, I knew that Medium wasn't working for me on three levels, and because I am compelled to write, I made peace with writing there for no payment.
I published that I would never charge for the subscription - which I won't. To me, it's too complicated to write one thing for those who want free subscriptions and another for those who pay.
So while I did put a note down asking for those who could to contribute something to Ko-fi, I was most surprised to find people actually paying for my subscription through Substack. I was even more surprised to find more and more people subscribing to me. To me, it's been a very pleasant surprise.