This element of music assumes importance in the development of musical composition because of its central and continued use.

The element you're referring to is likely melody. Melody is a sequence of single notes that is musically satisfying. It is the main theme of a song or piece of music and is often repeated or varied for the purpose of development in a composition. Melody is important in the development of musical composition because it is the part of the music that most people will remember and hum or sing. It is the central and continued use of melody that helps to create the mood, character, and structure of the piece.