The are trying to determine how many points each team will score and what the point difference will be. People can bet anywhere on the line (to win, to cover, to lose, etc) so the odds makers need to get the score differential pretty close. If they just picked a large spread then everyone would just bet to not cover the spread. The line changes as bets are made and as players are injured in practice but it is a estimation on how the think the game will turn out.