Why ECS pipeline needed Task Definition while Service already have the task definition assigned in AWS?

on bitbucket, the config is required to use both ‘service’ and ‘task definition’, I think this is redundant because the service already assigned with task definition, or we have an explanation?

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// https://bitbucket.org/atlassian/aws-ecs-deploy/src/1.2.0/README.md
  - pipe: atlassian/aws-ecs-deploy:1.2.0
    variables:
      AWS_ACCESS_KEY_ID: $AWS_ACCESS_KEY_ID
      AWS_SECRET_ACCESS_KEY: $AWS_SECRET_ACCESS_KEY
      AWS_DEFAULT_REGION: 'us-east-1'
      CLUSTER_NAME: 'my-ecs-cluster'
      SERVICE_NAME: 'my-ecs-service'
      TASK_DEFINITION: 'task-definition.json'

Source: Docker Questions