Why does China want to build a kilometre-long spacecraft?

Space elevator platform? Big cargo ship? Weapons platform? Artificial Space island?

They want to operate in space with at-scale projects. They have the intention and enthusiasm to do so. A better question is: why wouldn't you want a large space platform you can move around? Especially if you want to visit faraway places. Like Mars. Or even closer ones like Luna.

Economies of scale.

Think about the Portuguese caravels that were first to circumnavigate the World, and how tiny they were (roughly 100 tons). We don't bother with such small ocean-going ships today because they are too uneconomical to use. We use ships today that are a thousand or more times larger - around 200,000 tons.

Because they are watching Foundation?!