You can create groups and users using useradmin console. System user not at all required to create groups and users in repository.
//Why system user was introduced in AEM 6.x versions:
Use of admin session and admin resource resolver through ResourceresolverFactory is now deprecated, that’s why from AEM 6.1 Adobe forces developers to create system users and map them to Service User Mapper in Felix Console.
Prevent excessive use of administrative JCR Sessions and ResourceResolvers.
Allow services access to ResourceResolvers and JCR Sessions without requiring to hard-code or configure passwords.
Allow services to use service users and/or system user which are specially configured for service level access.
For more information on this system user topic see the below link: