What is the role of a promoter?

Promoter plays a key part in the initiation phase of transcription. Its primary role is to signal where the transcription process begins on a DNA strand. 

Promoters are composed of a specific sequence of nucleotides that relays specific information to RNA polymerase enzymes, telling it where to initiate transcription. It also allows the enzyme to attach to the DNA strand, provides a site for RNA to bind to the DNA, and indicates which strand is to be duplicated.