Purpose
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 a priori (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 will have 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. In order 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 will often revise their odds in order to manage their event risk.
Read more about this topic: Spread Betting
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