Democratising the basics
Titles, meta descriptions, canonical tags, XML sitemaps, schema output, breadcrumbs, readability prompts — Yoast packaged all of it into an interface that sits directly inside the editor where content is actually being written. That placement decision is the whole product.
Professional SEOs sometimes dismiss the traffic-light scoring as reductive. That misses the point: the plugin's audience is not professional SEOs.
Open source
De Valk has been a long-standing contributor to the WordPress project itself, which is part of why Yoast integrated so cleanly with the platform's own direction of travel over the years.