Use a common language to avoid misunderstandings developing. Testing is clarifying misunderstandings and through structured conversations like Example Mapping for discovering unknown unknowns can help tease out the common language.
Utilising the common (the domain) language applies to programming too. Rather than tying names to framework-specific concepts like models or controllers, use the language that is familiar to the domain.
Domains in Domain-Driven Design represent a Bounded Context - an area of responsibility in which there is a common understanding and representation of a process (Use the language of the domain)
Use the language of the domain to communicate intent and avoid test overspecification