Docker Compose for WordPress with Data Persistence

docker-compose.yml

# Docker Compose for WordPress with Data Persistence
#
# Resources
# https://medium.com/@tatemz/local-wordpress-development-with-docker-3-easy-steps-a7c375366b9
# https://hub.docker.com/_/wordpress/
# https://hub.docker.com/r/_/mariadb/
# https://stackoverflow.com/a/39208187/648844
# https://github.com/nezhar/wordpress-docker-compose
#
version: "2"
services:
  my-wpdb:
    image: mysql:5.7
    volumes:
      - ./wp-data:/var/lib/mysql
    ports:
      - "8081:3306"
    environment:
      MYSQL_ROOT_PASSWORD: root
  my-wp:
    image: wordpress
    volumes:
      - ./wp-app:/var/www/html
    ports:
      - "8888:80"
    links:
      - my-wpdb:mysql
    environment:
      WORDPRESS_DB_PASSWORD: root

volumes:
  wp-data:

why do you declare wp-data in volumes but not wp-app?