All tributes in Age of Empires and Age of Empires II have a 30% 'tribute tax', which meant that if a player sends 100 units of any resource, their recipient will receive 70 units instead.The tax has two purposes - firstly, it simulates how resources become lost on the way in the real-world due to corruption, bandits, tariffs, etc, and secondly, it discourages players from tossing around huge amounts of resources to allies without any penalization. There exists technologies at the Market building, which lower or even expunge the tribute tax, such as Coinage and Banking, these technologies do not feature in Age of Empires III.
In the singleplayer campaign and skirmishes, neutral computers may request resources in exchange for an alliance with the player; allied computers may also request for resources from the player in order to build up their forces. Players in turn may request resources from computers, who generally comply unless they have none to spare.
In multiplayer, the most common use for tribute is to aid more resource-starved team players in bolstering their military or defense. The manual for Age of Empires II also suggests sending tribute to enemy players in order to buy off an impending attack; this is ill-advised as sending tribute to enemies not only makes the stronger but also makes them more likely to attack regardless given the sudden influx of resources at hand.