Failing to understand when to use the serve_locally option for css and scripts

I have some custom react components and i am using tailwind via a CDN.

What i basically do is initializing Dash like this:

Dash(..., external_stylesheets=[
            {"src": ""},

What would app.css.config.serve_locally = True change here? Would it preload the css files to assets when the server starts? (I Mean i can’t see any files in there when i start the server with serve_locally = True