The NFL wide receiver's job is to get to a position where the quarterback can throw the ball without the defender interrupting the play. It's a basic game of "keep-away" with an incentive to move towards one end of the field. Naturally, this concept has developed over time to minimize the possible variance of outcomes during a play. Receivers run to a point where the QB expects them to be. This gives the offense an advantage over a defender covering the play. The offense will have multiple pass-catching options on-field at the same time so pre-determined routes make it easier for a QB to anticipate how a play is unfolding, make the right decision, and complete the pass.
Summer School: Receiver route types and combinations
