When the Vegas oddsmakers like sportsbooks and casinos set the point spread, or the line, for an NFL game, their objective is to make sure the line attracts an equal amount of wagers on both sides of the bet, as this limits the exposure of the oddsmaker.
The opening line is an oddsmakers' best guess as to which point spread will get equal action from bettors on both sides. Because most NFL games are mismatches, oddsmakers use a point spread to balance out the betting action on the two sides. Otherwise, everybody would just bet on the favorites to win, and the bookmakers would be left to cover the difference.