Encontrei o seguinte nesta página wiki .
An access control list (ACL), with respect to a computer file system, is a list of permissions attached to an object. An ACL specifies which users or system processes are granted access to objects, as well as what operations are allowed on given objects. Each entry in a typical ACL specifies a subject and an operation. For instance, if a file has an ACL that contains (Alice, delete), this would give Alice permission to delete the file.
Para responder à sua pergunta sobre "por que eles são importantes?" Se você ainda não entende, se você não tiver, as permissões não existiriam. É assim que o Windows entende quem tem certos privilégios.