If I'm a "nobody" that is saying I'm not important in the world's eye. I'm not "famous" or known by the world. If you are a "somebody" that means people all over your country know you. Some famous people are well-known all over the world....most people would recognize their picture. The quotation you give as an example means everybody who is well-known and important is here (at this party or convention or celebration.)

When there are very important or famous people present within an industry or profession. If you attended a fashion show, you could say this in regard to the people there within the context of the fashion industry.