Esses contêineres existem para compatibilidade reversa com domínios baseados em NT4 ao serem atualizados para domínios AD do Windows 2000 e muitos outros motivos que são devidos à compatibilidade do NT4 com o Windows 2000.
O MS KB 324949 fornece uma boa explicação das razões por trás disso devido a chamadas api herdadas ("versão anterior"): link
In a default installation of an Active Directory domain, user accounts, computer accounts, and groups are put in CN=objectclass containers instead of being put in a more desirable organizational unit class container. Similarly, user accounts, computer accounts, and groups that were created by using earlier-version APIs are put in the CN=Users and CN=computers containers.
Users, computers, and groups that are created by earlier-version APIs place objects in the DN path that is specified in the WellKnownObjects attribute that is located in the domain NC head. The following code example shows the relevant paths in the WellKnownObjects attribute from the CONTOSO.COM domain NC head.