All the candidates hope to win the caucuses, giving them a boost of attention as they shift focus to the New Hampshire primary eight days later. But more than a win, the goal is to exceed expectations. Even if a candidate finishes second or third, he or she can claim a victory by noting they finished near the top and received more support than expected.
It's worth noting that a win in Iowa doesn't necessarily translate into a party nomination. In fact, the last time the ultimate Republican nominee won a contested caucus was 2000, when George W. Bush finished first. The winner of the Democratic caucus has fared better. In the last three contested Democratic nomination races, the Iowa winner became the nominee.