First past the post restricts choice for the voter

Under proportional representation there is no need to worry about who everyone else is (expected to be) voting for, so there would be no requirement to switch to a more popular party, for your vote to count. Under Fptp (winner takes all) your vote counts more if you vote for a party expected to be popular, this is why a duopoly of parties will usually emerge, in voting systems of this type.

Voters care more about national representation than local because the laws are made nationally. If our representatives are taken from constituencies this limits our ability, as voters, to influence the national outcome. It is difficult for a liberal (or any minority view) to ensure that their vote will be effective if representation is at the constituency level, their opinion can be smothered. Sometimes the minority view will be prejudiced or otherwise problematic in some way, in this case (often) the bigots are in the minority, but this is not a problem because over time people tend to become more enlightened so this minority bigotry will be held by fewer and fewer people. It is already out of the political mainstream and so is not able to accomplish any lawmaking. The serious problems (delusions) in politics are those bigotries held by many people, including the political establishment. Under Fptp there is little ability (for voters) to diffuse popular bigotry, beliefs common to the main parties. Proportional representation is better at removing delusions held by (both) leading parties.

It is better not to have our choice restricted to only a small number of parties, choice is better because enlightened parties can then do well. Fptp enables parties to be unaware of their bigotries and lack of true popularity, it gives them a false sense of popularity. The main parties might be surprised by what people actually want.