You want to use jest to write tests that assert various things about the state of a DOM. As part of that goal, you want to avoid all the repetitive patterns that arise in doing so. Checking for an element's attributes, its text content, its css classes, you name it. The @testing-library/jest-dom library provides a set of custom jest matchers that you can use to extend jest. These will make your tests more declarative, clear to read and to maintain.