What is the purpose of making a preferment for yeast doughs?
In addition to allowing fermentation to begin before the addition of fermentation inhibiting other ingredients, as @J.A.I.L. said in his answer, according to Bread Secrets, after explaining the benefits to flavor of a long, slow fermentation:


  A quick, warm fermentation will allow time for the yeast to produce
  enough carbon dioxide to raise the dough, but not enough time for the
  enzymes to work or for the development of the other flavour compounds.
  If we ferment our dough for too long, however, the gluten becomes too
  weak to hold the shape of the loaf. So artisan bakers generally use a
  preferment (pre-ferment, not prefer-ment!) as part of their dough.
  This allows the bread to gain flavour from the longer fermentation,
  but still maintain a strong gluten network for a good rise.