The origins of this tradition can be traced back to the ballets in Indian dance-drama. Traditionally, these song-dance sequences are considered to be an outlet of the intense expressions of the lead characters of the movie. So they are picturised on the lead characters.
During 1940s, visual media was more or less immobile focussing only on the facial expressions of the artists while the auditory media was heavily based on Indian Classical Music.
During 1950s and 1960s, when technology facilitated mobility, Indian film makers started looking for scenic beauties in Kashmir and the rest of India. The auditory media was still inspired from folk and traditional music.
During 1970s, with the advent of Elvis Presley on the western horizons, Indian movie makers had something new to mimic. So the visual media was dominated by movers and shakers kind of dance. The auditory media was also more western with instruments like guitar taking a dominant role.
1980s saw the real decay and degenaration of this medium. Movie makers ran out of ideas and locations and so they started relying on redundancy. This decade's song picturizations are generally characterised by lead characters...
1990s saw a new kind of songs called family songs[?]. This was first introduced by Sooraj R.Barajatya's block buster Hum Aapke Hain Koun[?]. These kind of songs generally include the whole family dancing on a wedding night or the whole family displaying their love for one another.
2000s are seeing a new renaissance with a young generation of movie makers like Farhan Akthar[?] on the scene.