This is a great question. When educating children, we need to think about the totality of a child. We want children to grow up to their fullest potential and thrive as citizens. For this to happen, we cannot think of education as only subjects like Math, Science and English. The arts, music and even playing are essential in the development of a child. As for reasons, let me list three.
First, arts and music are essential in education, because it develops the creative areas of a child. This creativity is central to intellectual development. Later on in life, this creativity can be the impetus for great innovations.
Second, play is important, because it allows children to develop socially. In elite private schools in New York, play dates are central in determining a child's readiness for school. The social ramification later in life are also great.
Third, music allows a child's brain to develop maximally. For instance, playing piano develops the child's left and right side of the brain.