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.