This is a very good question. 1 emphasize "already , already happened ". 他都走了。He has already gone. 他都来了. He has already come. 我都吃饭了。I have already had my meal. 2. emphasize extent. 我都不明白这个问题。 连小孩子都会这个问题。even kids can answer this question. 3 emphasize all 他们都是我的朋友。

请问是在什么情况下用？

All of those men are friends of mine（他们都是我的朋友）. They are my friends.（他们是我的朋友）. 都≈All.