Looks like an interesting idea. Other than PHP-hate, why would someone choose this over something like Drupal?
Outside of being written in TypeScript with more modern conventions, the fact that Payload borders on an application framework is what would make someone choose it over something like Drupal.
Feature-wise, this commonly means reusing Payload's auth in your own app(s), defining function-based access control rather than RBAC, swapping in React components into the admin UI, and more.
But another HUGE reason is that when we built Payload, we tried to keep its own internal conventions as close as possible to just regular JS / TS. If you know JS, you know Payload. With Drupal and even WP to an extent, you need to know how _their_ conventions work. It's all very specific to the platform and the Payload team has always hated that. Devs don't want to learn CMS - they want to learn the underlying language, and Payload embraces that to its core.