The main idea behind spread betting is for the bookie to create a favourable market on either side of a wager. The reason for this is to create a series of events where participants in the sports betting procedure are willing to wager on either side, not just to win. This is particularly true of events that might be biased towards one team or the other. A good example of this is with sporting events where one particular team might have a history of being a dominant team while another might be a historically weak team who consistently loses. Individuals betting on this type of event would be more likely to favour the historically better team, to such an extent that betting on the other team could be conceived as a poor choice, thus the odds in favour of the weaker team skew in such a way that an individual placing a free bet on them and managing to win would end up with far more money than had they bet on the historically stronger team.
Spread betting generally evens the market out towards an equal number of participants on either side of the wager, thus allowing the bookie to create a market by accepting wagers on both sides of the spread. The bookie will charge a commission and act as a counterparty for every participant. As a result, the bookmaker is not necessarily concerned with the actual outcome of the event. Instead, his primary focus is on making sure people are spread betting on equal sides of the equation, with his profit coming from the commissions.