Varying snap counts is a great way to keep the defense from getting a great jump at the snap, to manufacture five yards in a key situation, and to gain a "free" play in which the defense has jumped offsides and the offense can take a shot down the field without fear of an interception or incompletion.
To take advantage of a "free" play, many teams now plan for the free play. Though teams may execute it differently, it works like this: the team has a called play, and a "free" play. If the defense jumps and the center snaps the ball, the receivers run streaks down the field, and the quarterback throws to one of them (the "free" play).