Because it turns pdf generation into a programming language, hence you can automatically generate invoices with little hassle.

Typst and its json based templating makes it easy to create good looking invoice PDFs for free.

The open-source solutions we tried before were annoying (Used LaTeX, wkhtmltopdf, weasyprint in production and evaluated several more). To avoid that pain we asked some commercial PDF generator software for a quote, but their per document pricing was insane.