Why missionaries and diplomates are given immunities and previliges?

why missionaries and diplomates are given immunities and previliges?

I don't believe missionaries have any immunities or privileges, but foreign diplomats are given immunity so as to avoid starting a war with another country. Could you imagine if the president of Russia came to America, was speeding, and then had to pay a traffic ticket? "Hey president of Russia, give us your money." Well that could spark up a war.