Lack of oxygen, extreme temperatures, fast moving particles and radiation are among the dangers humans face when they venture into space. Robots are immune to some of these dangers and less threatened by others. We have commonly used machines as our eyes, ears and hands to explore and do work in other hostile environments, such as deep-water exploration of the oceans. Just because an environment is hostile to humans though, doesn't mean that every job is best done with a robot. Sometimes, we haven't built a robot that can do the job, the matter is urgent, or things don't go as planned.
Emergencies happen and they often require the flexibility of a human.
The recent tragedy with the Russian submarine, Kursk is a good example of a situation where humans actually got there first. The Kursk sunk in deep, frigid waters, but because the escape hatch was damaged, all machines designed for deep-water rescue, were useless. In the end, specially trained divers were required to open the escape hatch. Occasionally, the political situation calls for humans to accomplish the task. The space race between the United States and the Soviet Union began with the launch of Sputnik in 1957.