Political culture is important in comparative politics because it can be used to help us understand why different countries have such different political systems.
For example, one of the most important texts of comparative politics was Almond and Verba's The Civic Culture: Political Attitudes and Democracy in Five Nations. This work sought to understand the different political cultures of five countries. It found that there were very different attitudes towards politics in Italy and Mexico (where people didn't care much about politics and didn't see it as separate from other parts of live), Germany (where they see politics as separate but don't really care much) and the US and UK (where people see it as separate and care a lot about it). Keep in mind that this study was done fifty years ago and cultures may have changed.
Because people in the five countries have such different attitudes, political institutions and political life are very different. Thus, the concept of political culture helps us to understand why different political systems are the way they are, which is a major focus of comparative politics.