China is currently building the world's most advanced UHV grid. UHV lines are good for carrying power over long distances. The high voltage level reduces line loss. But they are very expensive to build.
China's load centers are in the coastal East and South, while the best wind, solar, and hydro resources are in the North, Northwest, and Southwest. Power (esp. RE) is being generated far from where it's needed most.
UHV lines provide a inter-region backbone to transport power hundreds or thousands of km across the country. The UHVDC lines are used for the longest distances, while UHVAC lines are used for shorter distances, often within the same regional grid.