Answer :
Needed to be established, if men were Angels then there would be no reason for a Government everybody would be perfect... He's saying that the Government is essential because of Human nature (at least some people)
The main idea that Madison addresses is why the US government is needed.
James Madison starts the paragraph by presenting a hypothetical situation "if men were angels..." he tries to convey that if men were perfect, the establishment of a government in society wouldn't be needed since there wouldn't any conflicts, crime, disorganization or injustices. However, we are "imperfect" individuals, we aren't all wise enough to peacefully create and maintain a society in the benefit of all, hence the government is needed.
Furthermore, according to one of the founder father of America, we don't only need a government that governs the people but also governs itself, this latter referring that government must be balance and check by its parts (The branches of the Judicial, Legislative and Executive).