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.
In an effort to have equal money on both sides of a wager, the sportsbook operator will move the point spread to attract money on the side that customers aren’t betting on. The odds for a point spread might change before the actual point spread. There are certain point spread numbers, like 3 and 7 in football, the sportsbook operators would like to avoid moving away from since they final score margin falls on these two numbers most often.
Point Spread: The point spread remains the favorite way to wager on pro football, regardless of how many new forms of wagering come on stream. It’s called the line or spread and it’s known as betting ‘sides.’ The common misconception is that Las Vegas sets the spread as its best guess at the margin of victory. But really, it's a number they feel that is a perfect balance and will see an equal number of people to bet the underdog as on the favorite. A negative value like -6.5 means that team is favored by 6.5 points. So deduct 6.5 points from their total score. A positive value on the same game would be +6.5 (add 6.5 points to their final score) and would make that team an underdog of 6.5 points. The favorite must win by at least seven points to cover the spread. The underdog can lose by six points and still cover.