It probably targets developers that learned web development while learning React. It is at least a good idea for them. I think jsx-email wants to be the jsx version of mjml.
Or anyone who wants a tempting system or anyone who wants to conditionally render html esp in a templating system they already use, integrated with components and libraries they already have defined.
To me, the first one, but I’m not convinced JSX is a good idea anywhere and I’ve been writing HTML for 30 years.