I don't have my exact sources to hand but tributaries were also sign of prestige as well as an economic bonus for Chinese dynasties. It is partially why the system of coexistence [and the tributes the Chinese paid to the Khitan and the Tangut] were such a bitter pill for some scholars in the Song period to swallow. Tributes varied by reign but they most often contained strategic goods as well as gold/silver - the early dynasties often obtained prized bloodstock [horses] from the steppe in tribute for example. Other gifts included precious metals, animal skins, weapons such as bows and swords, some such as the kingdoms of Korea would provide bolts of cloth even including silk. Chinese dynasties would then provide 'gifts' [strictly labelled as such] in return for the diligence of foreign courts, often including silk - a prized commodity throughout imperial China and one very much in abundance in core China. Many kingdoms were coerced into providing manpower annually to an imperial court, with men and women often provided for a life of servitude. Without referencing my sources I cannot comment further on the economic benefit of tributes in specific dynasties other than to say they obviously provided an abundance of strategic goods which otherwise might be denied a Chinese empire.