Why is it that there is a different music director and a different background scorer for the same movie in Bollywood?
First things first. Scoring Background Music (BGM) for a film is a different craft than composing songs because the entire thought process is reversed.
Songs are composed BEFORE they are picturised. A composer is relatively free to experiment. The function is to inspire the lyricist, the singer, the Director and the actors to take it forward through their respective crafts.
BGM is done AFTER the scenes are already picturised and edited. A background score caters to the visual and is relatively limited in its scope of application. The entire thought process is different.
Of course, both require competence in composing music and lend equal contribution to the film, and there are examples of people having success in both the disciplines ( A. R. Rahman, Illairaja, Pritam for Barfi, Salim-Suleiman and of the younger lot Sachin-Jigar), but traditionally in Indian cinema these responsibilities have been handled by different people.
One of the reasons is also the financial aspect. For a top music director, who can churn out hit songs day after day, the fee as a BGM composer will be much less, if you factor in the fact that BGM takes a lot of time. So it’s. It wise for him / her to do BGM and let go of making songs.
Another reason, from the filmmakers’ point of view is that they sometimes want to have a different ‘style’ for their BGM than the style for the songs. It’s like having different approach to shoot scenes, than the approach taken to shoot the songs. Songs are larger than life sometimes and different dramatically from the storytelling style of the film itself.
Even in Hollywood, where the songs are sometimes included in the film, the composers of the songs are separate from the person who scores the BGM for the whole film (except the Musicals of course, because Musicals use music throughout the film and it’s better to have the same composer).
*The question was by Rahul Thanvi & its reply was first published on Quora