Why are metals sometimes added to gold?

Answer: In order for the mixture of gold and added metals to become hard and suitable for processing.

Explanation: Gold is precious metal, it has a great value, and it used to serve as a means of payment, but also proof of wealth, status and power. Gold as such is soft, and in order to improve hardness it is mixed with another metal, such as nickel, copper, palladium, zinc, etc. For example, white gold is obtained with a mixture of pure gold and nickel, which gives the mixture a white color, and is sufficiently tough suitable for making rings. Based on the composition of the mixture that is the percentage of gold in the mixture, the number of karats is determined, which also means the value of gold itself. For example, the 999 gold has 99.9% of the gold in the mixture, which means 24 K (karats), and represents the so-called mixture with the most valuable percentage of gold, or the purest of which can be purchased.

Because solo gold isn't strong material. But with admixture it's stronger.