In a context where primaries are not mandated, why do some parties use them to select presidential candidates? In this paper, we argue that the use of primary elections is likely to reflect the existence and intensity of divisions within political parties. We focus on several characteristics of political parties and party systems that are often associated with intraparty divisions. We find that these characteristics are correlated with the decision of Latin American parties to use primaries in selecting their presidential nominees. In particular, we find that coalitions and large, centrist parties are more likely to hold primaries, whereas primaries are less likely in institutional settings that facilitate exit from existing parties.