The general purpose of spread betting is to create an active market for both sides of a binary wager, even if the outcome of an event may appear prima facie to be biased towards one side or the other. In a sporting event a strong team may be matched up against a historically weaker team; almost every game has a favorite and an underdog. If the wager is simply "Will the favorite win?", more bets are likely to be made for the favorite, possibly to such an extent that there would be very few betters willing to take the underdog.
The point spread is essentially a handicap towards the underdog. The wager becomes "Will the favorite win by more than the point spread?" The point spread can be moved to any level to create an equal number of participants on each side of the wager. This allows a bookmaker to act as a market maker by accepting wagers on both sides of the spread. The bookmaker charges a commission, or vigorish, and acts as the counterparty for each participant. As long as the total amount wagered on each side is roughly equal, the bookmaker is unconcerned with the actual outcome; profits instead come from the commissions.
Because the spread is intended to create an equal number of wagers on either side, the implied probability is 50% for both sides of the wager. To profit, the bookmaker must pay one side (or both sides) less than this notional amount. In practice, spreads may be perceived as slightly favoring one side, and bookmakers often revise their odds to manage their event risk.
One important assumption is that to be credited with a win, either team only needs to win by the minimum of the rules of the game, without regard to the margin of victory. This implies that teams in a winning position will not necessarily try to extend their margin—and more importantly, each team is only playing to win rather than to beat the point spread. This assumption does not necessarily hold in all situations. For example, at the end of a season, the total points scored by a team can affect future events such as playoff seeding and positioning for the amateur draft, and teams may "run up" the score in such situations. In virtually all sports, players and other on-field contributors are forbidden from being involved in sports betting and thus have no incentive to consider the point spread during play; any attempt to manipulate the outcome of a game for gambling purposes would be considered match fixing, and the penalty is typically a lifetime banishment from the sport, such is the lack of tolerance for gambling in sport.