...and how is it worth twice as large a share as Patreon takes? (10% after the payment processor's fees, vs 5%)

We think Patreon is awesome. It's a great general purpose tool for supporting creators, and the fact that it is so successful shows that people are willing to pay for stuff they value.

Substack is focused on letting writers start their own publication - top to bottom - with subscriptions as an integral part.

To add to Chris's point (this is Hamish), I think Substack is just really simple. As a writer, I like the idea of not having to think about building and designing my own site, or coming up with various payment and rewards tiers, so I can focus almost all of my time on writing.