In the Catholic Church, what is the explanation for having, and the history behind Principal Co-Consecrators?

When researching for an answer, I wanted to provide a link to Pope Bl. Pius IX . He was ordained Bishop *28 Oct 1919* by *Principal Consecrator*: Archbishop Aleksander Kakowski † Archbishop of Warszawa {Warsaw}. I was surprised to see other two *Principal Co-Consecrators*. Isn't one Bishop enough to ordain and consecrate a candidate a Bishop? *The question* What is the purpose of the *Principal Co-Consecrators*? In Church history, what is the explanation for having, and the history behind having *Principal Co-Consecrators*? Who decreed it and when?