Initialize configuration in application factory on flask

I saw some sample codes, but I can't understand some part of these codes.

Here is the content of sample codes:

• Flask application structure:

.
├── app
│   ├── __init__.py
└── config.py

• config.py:

    import os
    basedir = os.path.abspath(os.path.dirname(__file__))
    
    class Config:
        SECRET_KEY = os.environ.get('SECRET_KEY') or 'hard to guess string'
        # ...
        
            @staticmethod
        def init_app(app):
            pass
    
    
    class DevelopmentConfig(Config):
        # ...
    
    
    class TestingConfig(Config):
        # ...
    
    
    class ProductionConfig(Config):
        # ...
    
    
    config = {
        'development': DevelopmentConfig,
        'testing': TestingConfig,
        'production': ProductionConfig,
        'default': DevelopmentConfig
    }

• app/init.py

    from flask import Flask, render_template
    from config import config
    # ...
    
    def create_app(config_name):
        app = Flask(__name__)
        app.config.from_object(config[config_name])
        config[config_name].init_app(app)
            # ...
        return app

I want to know the usage of these lines:

• config[config_name].init_app(app)

• def init_app(app):
      pass
  

If I ignore it, what would happen?

Read more here: https://stackoverflow.com/questions/66273938/initialize-configuration-in-application-factory-on-flask