Why did people learn martial arts in Ancient China?

There should be two major reasons. The first is to keep fit, which is the most important one. The other is for self-defense.

I don't think staying fit would be as important as fighting back. Many of the famous martial arts groups were enlarged by people wanting to fight back against the government. If it was not Ancient China and it was present china I would be inclined to agree with Dierser, but that is not the case. Self defense and offense were the biggest reasons to learn martial arts in Ancient China.