What is the purpose of the incus?
The incus is there in humans because it is there in our mammalian ancestors, and it is there in our ancestors because the mammalian three-bone ossicle system was inherited from early amniotes.

The synapsid ancestors of mammals had the ancestral jaw articulation between the articular (later called malleus) and quadrate (later called incus). This is the bottom image below. There are fossil species with a transitional form in which there are two jaw joints (e.g., Pachygenelus). In extant mammals, the jaw joint lies between the dentary and squamosal (part of the temporal bone), while the old articular and quadrate are part of the ossicle system (top image below).

There is a lot of material on the web about this:


http://crompton.oeb.harvard.edu/publications/transformation-quadrate-incus-through-transition-nonmammalian-cynodonts
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3552421/
http://www.zoology.ubc.ca/~millen/vertebrate/bio204_labs/Lab_3__Skull.html


So in short, the incus is there because there was no other way for the mammalian ear to evolve from the ancestral system. It doesn't have a purpose per se, except that it's always (at least going back to jawed fishes) been attached to the stapes and articular (malleus).