To effectively manage infrastructure and environments, at some point, you realise multiple AWS accounts are necessary.

You might want to separate Dev and Test Accounts from Production ones ( so that billing and quotas are not mixed up, and maybe only few people, or CI Pipelines, can touch Live apps and so on), or you might want different departments or business units to have their own account and manage their own AWS services.

AWS accounts are natural boundaries for permissions, security, costs, and workloads. On the other hand, the more accounts and environments you have, the more risks and vulnerabilities ( due to loose or wrong configuration ) you are exposed, and the more management complexity you have.

That is why AWS Organizations is useful: to consolidate multiple AWS accounts, organise them into hierarchies, and centrally manage them.