To protect from the sun exposion just the same as anyone else.
Secret Service agents sometimes wear sunglasses to keep the sun out of their eyes, so they can increase their ability to see what people in the crowd are doing.
Maybe in the movies, they are wearing sunglasses all the time to look more cool and stylish. But in reality, agents do not always wear sunglasses.
Some people also think that they have technology built into sunglasses to help them at work, like James Bond or something. But we don't konw it is true or not.
I discovered this too. In most movies, secret service people tend to wear sunglasses. I think it is really cool. On one hand, they need to have many outdoor activities. Sunglasses can provide them better vision by reducing the glare and strong sunshine and protect their eyes from damages. On another hand, I think sunglasses are really good tool to play cool. They usually wear dark sunglasses, which look like dark curtains so that others can see clear their faces and expressions. For me, sunglasses are good eye protection tools and a fabulous accessories. I love sunglasses.