What is the purpose of an abrader in flint knapping?
The purpose is to prepare a flake for improved manipulation.

Abrasion is a process also referred to as 'raking': 


  Raking is the action of carefully dragging a course abrader or other device to remove "micro" flakes from the edge of a biface or preform to change it's [sic] shape or give support to an edge before actual abrading is done prior to percussion or pressure work.
  source
  
  Edges being worked [especially while soft hammer knapping] must be ground dull prior to flake removal. This dulling helps prevent edge collapse. A piece of sandstone, very soft limestone, or other soft rock may be used to dull the edge.
  source
  
  The process [of lithic reduction] also involves frequent preparation of the edge to form better platforms for pressing off flakes. This is usually accomplished with abraiders made from a coarse-grained stone such as basalt or quartzite.
  source


Essentially, the abrasion allows for the force to be distributed more evenly and controllably.