As on Wikipedia
Production music (also known as stock music or library music) is the name given to recorded music that can be licensed to customers for use in film, television, radio and other media Oftentimes, the music is produced and owned by production music libraries.
We hear production music everyday on TV or radio ads,documentaries,short films. the music used in movies is called “Background Score”that is totally different genre i will write the dedicated post on this subject. The purpose of this music is to support and elevate the content.