Although we typically think of ethnocentrism as something that is very bad, it can at least arguably have some good consequences for group life.
Ethnocentrism is generally seen as a bad thing because it leads to prejudice and hatred of other groups. Ethnocentrism is the belief that our own ethnic group is different from, and in some way superior to, other ethnic groups. This can lead us to actively despise other groups and, at times, to try to harm them. Thus, we can say that ethnocentrism led to such atrocities as the Holocaust or, on a lower level, to things like apartheid in South Africa or slavery in the United States.
However, we can at least argue that ethnocentrism can have a good side. In general, it can be hard for modern societies to stick together and for people in such societies to feel much of a connection with one another. When this happens, there can be problems such as crime or civil discord. Ethnocentrism can help to prevent this. If people have the attitude that their group is better than others, it can give them a sense of identity that binds them to one another. In that way, ethnocentrism can lead to greater cohesion within a group.