There seems to be a lot of people that are confused and think that the actual probability of a sport betting event is equal to the odds offered by online sportsbooks, and that's incorrect.
When you calculate probability - whether it's based on experience (basically guessing), or through mathematical procedures - you end up calculating which team (or player) will win the event, and you can judge if the lines offered by online sportsbooks will benefit your bet or not.
On the other hand, online sportsbooks try to make both sides of the bet equally attractive in order to distribute the cash. In the end, if the book did everything right they will be able to collect their part (commonly called €juice€) and pay the winners. In the end, the odds and lines are nothing more than numbers which can be manipulated at any given time by those in charge in order to distribute the bets as equally as possible between both sides.
Online sportsbooks don't care which team will win, and their numbers most certainly not reflect reality; that's where professional gamblers and casual players begin differentiating themselves.
Understanding this allows the bettor to define the probability of a certain outcome and then decide whether the lines, spreads and other odds are worth the risk.
In order to properly calculate probability a vast amount of factors must be considered, including individual player's performance, training, teams win/lose record, field where the event is taking place in, pressure to win and incentives, changes in team line-up, injuries and weather among others.
Besides taking into account those variables, expertise is required as well. It takes time to understand when to follow your guts, and those who manage to master this skill can find out profit where almost everyone else thought there was none.
As for online sportsbook odds, don't ignore them completely. It's true that their purpose is not to predict the real outcome of the game, nor accurately reflect the probability of one team winning, but they help understand the expectations of other gamblers - which sometimes can save you from making a mistake if you forgot to take into account one of the variables when making your analysis - and you'll also get to know if there are some €sharks€ betting on one side if there are reverse line movements (or extreme gaps in the same direction it was heading).
Hopefully this will help many understand that using odds provided by online sportsbooks as a point of reference to know who will win isn't the proper way to bet; and hopefully it'll help you profit more constantly.