Ethnocentrism causes problems both for individuals and for societies as a whole, especially in our contemporary highly globalized world.
In much of our work and even daily lives we encounter people from many different ethnic backgrounds with different cultural assumptions and mannerisms. To work with them, we must understand their cultures. For example, in Navajo society, looking people directly in the eyes is considered rude. Knowing this means that you will not be offended if Navajos look at the sky while speaking to you and you also can not stare directly into their eyes as a sign of respect. The more you know about other cultures and their manners and beliefs, the more you can work well with people from many different cultures and avoid offending them. Perhaps even more important, this sort of sensitivity to other people's cultures allows you to make friends with people of very different backgrounds from your own. Your own personal world is enriched by your ability see things from other points of view.
On a global level, not understanding cultural differences can lead to diplomatic incidents or even wars. Often behaviors accepted in one society are considered unacceptable in others. On a level more serious than just manners, western ethnocentrism and ignorance of cultural issues in the Islamic world reflected in the Sykes–Picot Agreement of 1916 led to many of the current tensions in the Middle East.