It is often said that everyone loves a winner, and people like to root for a winner. The crowds at sporting events are larger when the home team is a winning team because people would rather be fans of a winning team and not a losing team.
Unfortunately, we have taken the same attitude to politics. Many people are so desirous of voting for a winner that they vote for the candidate they think will win, rather than the candidate they want to win. And so some voters will describe their own votes as "wasted" merely because their candidate failed to win.
Pundits sometimes describe this as "momentum," because popularity can be a self-fulfilling prophesy. Candidates that are perceived as successful, popular, and potential winners, become more popular and more successful simply as people hop onto what they think is a winning bandwagon.
In fact, it is arguable that Hillary Clinton's entire campaign to date has been based on this phenomenon. With an early lead in public opinion polls (which was due mainly to name recognition), her campaign has sought to present her success as inevitable, so that any vote for any other candidate is futile (and wasted).
This phenomenon also explains the importance of the early primaries in Iowa and New Hampshire. Candidates that do well in those two small states may be able to coast to national victories simply because they are perceived as winners, which causes more people to vote for them in later primaries.
Unfortunately, the mainstream media (and most pundits) actually encourage this kind of thinking by covering political campaigns as though they were sporting events. They rely on polls (and often just guesswork) to talk about who's behind and who's ahead, what are their tactics and strategies, and what are their chances, while hardly mentioning at all the real policy differences among the candidates. So most voters will go to the polls with all sorts of ideas about who's likely to win, but very little idea about which candidate stands for policies that are favored by the voter.
The bandwagon effect is also fueled (or perhaps magnified) by our "winner take all" political system, in which only the candidates with the largest plurality of votes (a majority is not needed in most elections) is elected. This discourages votes for third-party candidates, and also denies any political representation to voters representing sizable minorities of the population. (More about this another day.)