Without PROGRESSIVE OVERLOAD your body does not need to adapt and therefore will never get bigger or stronger beyond a certain point.
…A common goal for strength-training programs is to increase or to maintain one’s physical strength or muscle mass. In order to achieve more strength as opposed to maintaining the current strength capacity, the muscles (see skeletal muscles) need to be overloaded which stimulates the natural, adaptive processes of the body which develops to cope with the new demands placed on it.
Progressive overload not only stimulates muscle hypertrophy, but it also stimulates the development of stronger and denser bones, ligaments, tendons and cartilage. Progressive overload also incrementally increases blood flow to exercised regions of the body and stimulates more responsive nerve connections between the brain and the muscles involved.
Conversely, decreased use of the muscle results in incremental loss of mass and strength, known as muscular atrophy (see atrophy and muscle atrophy). Sedentary people often lose a pound or more of muscle annually…
This is a good article if you want to learn a bit more 🙂