Power
You have a certain amount of power when representing your country, but at the end of the day, it's your boss, the President, who's going to make the toughest calls. You're there to provide them with the details specific to the topic, whether it's something to do with foreign policy, a trade negotiation, or human trafficking. Then they get to make the decision.
That being said, how you present your research, findings, observations—whatever you do in your job—to your superiors can greatly influence the people of the country in which you're serving. You make recommendations based on your knowledge, and if you've become known as a trustworthy and positive influence, the ambassador or even the POTUS may very well take your ideas and run with them.
So be careful what you say, how you say it, and to whom you say it; your proposals, endorsements, and ideas about what's best for your country within the host country will probably be taken seriously and could have a deep impact on people's lives.