It's always worth learning a level lower from what you do, so if you write frontend UIs with React it's worth learning how DOM manipulation works with vanilla JS to understand the intricacies and appreciate what the library abstracts for you.
If you start writing a site in vanilla JS you'll eventually end up writing your own UI library basically reinventing the wheel which is fine as a learning experience and personal projects but maybe not the best choice if you're working on a large scale project with many developers since UI libraries and frameworks provide a common set of patterns as convention to follow that makes it easier for everybody to be on the same page.
If you start writing a site in vanilla JS you'll eventually end up writing your own UI library basically reinventing the wheel which is fine as a learning experience and personal projects but maybe not the best choice if you're working on a large scale project with many developers since UI libraries and frameworks provide a common set of patterns as convention to follow that makes it easier for everybody to be on the same page.