Kishore Mahbubani, a scholar on government policy, claimed (YouTube link) that China is trying to develop Africa more than any other country. What benefits would China gain by developing Africa geopolitically?
A market, a manufacturing base, a source of raw materials, and a network of allies
Not everything that a country does is always defined by narrow self-interest. However a more economically developed Africa would be an expanded market for China to trade with.
African countries are already major suppliers of raw materials. China would like to obtain these at preferential rates.
China itself currently has a large manufacturing sector, and is competitive compared to Europe and America in part because wages are lower. As China becomes richer its people will expect higher wages and so China will look to Africa to make consumer items for its people.
Finally by being engaged with the development of African countries, China will make political allies, which has the potential to be useful in future international disagreements or conflict.