Compared to the Forge it replaces, the Mint gives +10% in the city it is constructed in.
Mali is a Spiritual and Financial civilization - two traits that can raise a lot of money. Forges are built in most cities, if not all, usually, and are aimed for early on. The Mint is just a Forge that makes cities raise more money. These three together go hand-in-hand spectacularly.
Thanks to the Mint, running an empire at 90% or less science raises substantially more money than usual - as most cities have Forges, and 90% or less science makes every city raise cash, you'll raise much more. Mansa Musa's commerce boost makes this even better.
At the height of its power, the Mali Empire had a trading empire that stretched across Africa and into Europe, the Middle East, and Asia; the great wealth of the Mali Empire in large came from the rich gold mines of Bumbuk and Bure. Once the gold was extracted from the mine and purified, it needed to be shaped into a size and form that was easy for the Malinese to transport and to trade. Gold could be formed into large blocks or bars, but these were extremely heavy and cumbersome, and quite awkward to transport on horse/camelback. They were also so expensive that only governments and the very rich would be able to afford them, and so that form was usually reserved for large quantities of gold that were to be transported by ship. For smaller and more convenient uses, gold was formed into coins.
A place where money is created is called a "mint." At a mint, the gold is melted and poured into a standardized coin form - or, in earlier times, it might be manually pounded and cut into shape. The coin is usually decorated with the ruler's head or some other recognizable symbol. The value of the gold coin is dependent upon the purity of the metal in the coin, and its weight. Throughout history the unscrupulous have attempted to pass off light or impure coins to unsuspecting purchasers. If widespread, this practice can seriously damage trade and cripple a nation's entire economy. To maintain a reputation for honest dealing, the Malinese needed to use fairly sophisticated measuring tools to ensure that their coins maintained a consistent value. This in turn spurred advancements in science and technology.