In a properly functioning parliamentary democracy, voters can do much more than cast a vote from time to time. They should be able to hold their government to account and, if they decide they don't like it, replace it with another - in effect, a government in waiting.
If the system works well, that government in waiting will have been identified in advance, and the voters will have had the chance to compare what it offers in prospect with what has been delivered by its predecessor.
It doesn't always work like this, of course. In some systems, the voters find it hard to get rid of the government, let alone identify a credible successor. In post-war Italy repeated elections were held but voters could never get rid of the Christian Democrats, not because they were so popular but because the opposition parties were so fragmented.
It was often said - with some justification - that this was a particular weakness of proportional representation systems. But it has been the particular genius of New Zealand voters that we have managed to secure through MMP the advantages of a more representative Parliament without losing the essential choice between right-of-centre and left-of-centre governments.
There is a further advantage of a system which produces credible competing contenders for office. It usually compels the contenders to vie for the support of centre or uncommitted opinion. It is, in other words, a force for moderation in our politics.