Vegas wants an equal number of bets on both sides of a betting line. In a perfect world, both sides have the same amount of bets to cover the winnings on the other side. Vegas just wants the commission for facilitating the bet.

The spread is not predicting the expected outcome. The spread is created in an attempt to even out those bets to prevent taking losses. Big fan-base teams (e.g. Notre Dame) will typically have higher spreads because of the large number of fans betting on their team.