The current Ontario election is a perfect situation demonstrating why preferential voting is needed.
The polls show that this election is not about which candidate or government voters want, it is about which one they do not want. According to a recent poll, a third of total voters, and half the voters for candidates of one party are voting only to prevent someone from being elected.
First past the post voting makes it difficult to achieve this primary objective of a large part of voters. There is no built in mechanism to ensure that voters can prevent someone from being elected if voters don’t want them elected. In fact, elections such as this are the ideal conditions for the votes of the electorate to have exactly the opposite effect from what was intended. With voters casting strategic ballots but without accurate information about the intentions of other voters in their own riding, and possibly being misled by province-level polls and misleading information from parties, it is possible to accidentally elect the very candidate that the majority was trying to avoid electing.
If in your riding the majority wants to prevent Candidate A from being elected, and the majority think that Candidate B is much better than Candidate C, there is no way for strategic voters to communicate to each other that Candidate B is their preference, and many may vote for their less preferred Candidate C, mistakenly believing that this minimizes the risk of accidentally electing Candidate A.
Proportional representation has even less of a mechanism for ensuring that a particular candidate or government is not elected. A majority consensus not to elect someone is not sufficient to stop them being elected, only a small percentage of votes is required for someone to be elected. Even with a strong feeling against an entire party, the results will be roughly the same as the absence of a strong feeling against them: their supporters will vote for them and there will be a stalemate from the voters, after which politicians will decide among themselves who governs, making deals based on their own self-interest.
Preferential voting is the electoral system that lets voters pass a negative judgment and effectively prevent someone from representing you if that is the wish of the majority. It does this without guesswork as to the intentions of others and without requiring access to information that is currently very difficult to get. It prevents the accidental election of the candidate that the majority was trying to stop. And it does all this without sacrificing your ability to vote for your first choice. It is equally applicable for a positive vote or a negative vote, for a vote for or against a party or for or against a candidate.