The phrase "obligatory part" refers to something that is necessary or required. It is something you have to do or include as a part of a particular activity, event, or task. Full definition
West Bank (CNN)- Boarding a bulletproof bus to travel through one of the Middle East's perennial hot spots may not be most people's idea of a fun vacation activity, but for a growing number of Christians visiting West Bank settlements, it's becoming an obligatory part of their visit to the Holy Land. (religion.blogs.cnn.com)