goal of oddsmakers is to get even amounts of money bet on both sides of the line. If they can do that, the casino/sports book is guaranteed to make money. The reason is the sports book takes a commission from all the winners.

The reason lines move is when there is an imbalance in the money bet on one side vs the other.

To the extent a line is a prediction it’s only an attempt to predict what line will draw even money on both sides. It is not a prediction on the actual outcome of the game.