One way in which Stevenson's contribution relates to the nature and function of fiction is in outlining the contours of the genre.  Stevenson actively acknowledges other voices in defining the art of fiction.  In this process, he adds his own understanding to both the genre as well as the artist.  One of the contributions in this regard is to suggest that the artist is both mirror and looking glass to the world:

The novel, which is a work of art, exists, not by its resemblances to life, which are forced and material, as a shoe must still consist of leather, but by its immeasurable difference from life, which is designed and significant, and is both the method and the meaning of the work.

Stevenson suggests that the novelist, the fiction writer, is able to embrace the life of what is around them.  Yet, at the same time, pivot to a construction of what might be and what can be.  Stevenson's contribution to the discourse is to present the transformative quality of fiction and the fiction writer.  

Within this discussion, Stevenson advocates a sense of imagination that is intrinsic to the fiction writer.  Stevenson makes clear that one of the artist's primary responsibilities is to capture the spirit of imagination that allows fiction writing to embody the premise of what can as opposed to being strictly tethered to what is:

There never was a child (unless Master James) but has hunted gold, and been a pirate, and a military commander, and a bandit of the mountains; but has fought, and suffered shipwreck and prison, and imbrued its little hands in gore, and gallantly retrieved the lost battle, and triumphantly protected innocence and beauty.

Stevenson's contribution to the nature and function of fiction is to illuminate the role and power of imagination. The fiction genre itself is situated in this domain.  Stevenson wants to assert that its true value is affirming how individuals can possess the capacity for envisioning what might be out of a world steeped in what is.  Stevenson's work is a testament to this, for his constructs are grounded in the need for imagination both the part of the author and the reader.  This contribution becomes essential as it testifies to the power of fiction and its impact on the audience.