Beyond strategic reasons mentioned elsewhere, the main ones are (a) local candidate of desired party is unqualified&#x2F;unpopular&#x2F;etc and (b) local candidate of less desired party is more qualified&#x2F;more popular&#x2F;etc. (Seeing as &quot;popularity&quot; is, for better or worse, a pretty important quality in a political candidate; this includes &quot;I went to the same high school as candidate X, they seem like a good person.&quot;)<p>This happens easily in more-than-two party systems like Canada or the U.K. Since each party overlaps with another a bit on certain issues, and many voters only seem to feel strongly about a few issues, those voters are ideologically okay with more than one party. Even if they have a &quot;favourite&quot; party, they will consider the candidates of those other parties as well.