Why do I need to return the sealed medical envelope to the US Embassy?

Normal, because there is a backup of the embassy.

An embassy is a permanent diplomatic representative office of a country in the capital of a country with diplomatic relations. The embassy represents the interests of the whole country and is fully responsible for bilateral relations. Curators are usually ambassadors, but they can also be ministers or other diplomats appointed by sending countries. He was appointed by the head of state and performed his duties as the representative of the head of state.

The primary duty of the embassy is to represent the sending country and promote political relations between the two countries, followed by promoting economic, cultural, educational, scientific, technological and military relations. The embassy also has consular functions. Promoting relations between the two countries and non-governmental exchanges is an important duty of the consulate, but its main duty is consular work, such as safeguarding the legitimate rights and interests of its citizens and legal persons in foreign countries, issuing or extending passports for its citizens and issuing visas for foreign citizens.