`mix phx.digest` creates the manifest

mix phx.digest creates production ready assets: hashed, zipped and compressed.

In production, when you call:

Endpoint.static_path('asset_name.css')

This will look for a file in the priv/static directory and return the path for that file. But what you want is the hashed version of that file, because modern browsers are greedy cachers and will you use the use to bust that cache.

The static_path function can return a path that represents the cached version, but it needs to read the manifest, which maps the file to it's hashed, zipped and compressed versions.

mix phx.digest creates the manifest along with the hashed, zipped and compressed versions of all the files in the priv/static directory.