What is the significance of having 15 point of views (15 different narrators), what are some advantages and disadvantages?

Normally fifteen different narrators would be overwhelming. The text would be confusing and scattered. As I lay dying, however, is was written by William Faulkner. In his expert hands we get to soak up an array of narrative styles yet keep an intimate unity. Check out this from GradeSaver,

The structure of As I Lay Dying is powerful and innovative. Fifteen narrators alternate, delivering interior monologues with varying degrees of coherence and emotional intensity. The language is intense and highly subjective, with a recognizable change in language depending on the narrator. Each section falls somewhere in the range from confessional to stream-of-consciousness. The novel is a series of interior monologues, and through these fragmented passages we piece together the story of Addie Bundren's death and the transport of her body to Jefferson.

The narrative appears fragmentary, but the story demonstrates admirable unity: it is limited to the span of a few days, and the different sub-plots are logically and skillfully interwoven. Faulkner's innovation is in how we see this unified set of events: we are forced to look at the story from a number of different perspectives, each of which is highly subjective. In The Sound and the Fury, Faulkner made use of some elements of this technique. However, As I Lay Dying presents us with a far greater range of voices. Additionally, The Sound and the Fury provides a clearer distinction between unreliable and reliable narrators. Part Three of The Sound and the Fury is narrated by a man who is unmistakably evil, and Part Four helps clarify the novel through its use of a more objective third-person narrator. The voices in As I Lay Dying are more numerous and more ambiguous.