This paper describes the whole-system based model called Whole-electricity System Investment Model to quantify the benefits of demand flexibility. Whole-electricity System Investment Model is a holistic and comprehensive electricity system analysis model, which simultaneously optimises the long-term investment decisions against real-time operation decisions taking into account the flexibility provided by demand. The optimisation considers the impact of demand side response across all power subsystems, i.e. generation, transmission and distribution systems, in a coordinated fashion. This allows the model to capture the potential conflicts and synergies between different applications of demand side response in supporting particularly intermittency management at the national level, improving capacity margin, and minimising the cost of electrification. The impact and value of demand side response driven by whole-system approach are compared against the impact and value of distribution system operator or transmission system operator centric (silo approaches) demand side response applications and the importance of control coordination between distribution system operator and transmission system operator for optimal demand side response is discussed and highlighted.