Opinion polls can influence the voter's decision because they give an overview of the most likely outcome of the election. In this way, voters can change their votes to boost or prevent the advance of a particular candidate, a "useful vote".
For example, if the voter is going to vote for a candidate C, but the polls indicate that candidates A and B are technically tied and the voter does not want candidate A. If by examination candidate C has no prospect of victory, the voter may vote for candidate B, in order to avoid the victory of A.