DNS is required so objects within the domain can find resources. Like how is a client desktop supposed to know where the SYSVOL share is for domain.local unless DNS is there for it to look it up?
DNS is an essential part of ADS, bcz ads is designed for centralized resource management, so every users or computers must have a contact with domain server in order to be resolved when a request is raised for example when you try to use printer , your client machine will take the request to domain server to find the printer..
without dnd clients cannot resolve the names in domain which is an essential for ADS..