Buildings constructed since the early-to-mid 20th century typically have an outer wall with a cavity. Originally designed to better prevent water from coming through, since the 1960s and 70s these have often been filled with insulation - primarily for thermal performance.
Cavity wall Insulation can bring additional benefits by reducing carbon emissions - and improving health and indoor comfort.
Without proper insulation, properties will lose more heat in moderate and cold climates - resulting in greater energy use to warm them effectively. In hot climates, heat will easily come in, increasing the need for cooling. This all leads to higher energy bills.