In many voting systems, it is against the rules to poll the suffrage close to the election as it may sway their vote. They may either choose StrategicVoting?, or decide not to vote because of a perceived landslide. So, it is often undetermined in close votes who the victor is likely to be before the election.
Also, with SecretBallot?, since ballots are generally counted by hand, it takes anywhere between hours and days to know who the victor is. However, the people--or more directly, the press--want to know who won.
Therefore, since it is legal to poll people after they vote, the press conducts an ExitPoll. That is, when people leave the voting booth, they are polled to see how they voted. From this, a prediction of who won can be made before the actual preliminary returns are made.
But, since voting is usually spread over a large timescale (say over the course of a day), if the ExitPoll results are returned very early, people may not vote later on deeming it hopeless. Consequently, it is usually against the rules to report the results of the poll until after the poll closes.
In countries in Canada where no account for time zones has been made in the legislation, the results from the east coast are reported well before the west coast finishes voting. Consequently, voter turn out is lower in the west coast.