Typical use cases are implementing changes due to deprecated functionalities, cleaning up code (e.g. renaming variables, removing unused code, or removing debugging statements like console calls), or detecting secrets/passwords in files. Some libraries and frameworks even provide Codemod transformations to help upgrade a codebase when a feature is deprecated.
astexplorer.net is a great web-based tool to run codemods, conveniently combining all necessary views (i.e. original code, AST, applied codemod, and resulting code) in one window.
The example above uses recast as parser and jscodeshift as transformer and represents a simple/stupid codemod: reversing the characters of names of identifiers (e.g. variable/method names). jscodeshift is an exemplary tool for running codemods over JavaScript/TypeScript files.