1.) Well if you remember line 90 in Act 1 Scene 1, Iago says "an old black ram is tupping your white ewe". This animal imagery is distinct and is intended to shock the audience and create a vivid visual picture which is insulting and discriminatory. When Iago told Brabantio this, the audience hadn't met Othello yet so the audience might have been inclined to mistrust him at first which is exactly Iago's intention.
2.) Not too sure, but as Othello is already a controversial character being an honest and noble 'moor' of rank, by presenting him in a way that goes with the stereotypes of blacks being savages and uncivilized helps shape his honorable character later.
Hope this helps