AFAIK this allows you to use JS libraries like they had type annotations like fully typed TypeScript code. If they are provided for a JS library you get proper autocompletion support and lint checks, …
See also
- https://blog.mariusschulz.com/2014/05/19/using-typescripts-type-definition-files-to-get-tooling-support-for-plain-javascript
- Are typescript type definitions required?
- How do you produce a .d.ts “typings” definition file from an existing JavaScript library?