When people are asked whom they will vote for, they might not want to say. After all, other people might judge them, ask for contributions, or publish the answer. Suppose there are two candidates, randomly called B and T. Suppose, again for the sake of this hypothetical, that there is a slight stigma against people who support T.
The pollster says to them: "Please flip a coin. If the coin comes up tails, please tell us whom you like best. If it comes up heads, then always say T." That way, even if a person states an intention to vote for T, nobody knows for sure.