Quicksilver is won by roasting cinnabar, mercury sulphide, a brilliant red ore that was also the source of the pigment vermilion. The same mine therefore supplied a sacred colour and a running metal, and the conversion of a red stone into a silver liquid is a visible transformation that any competent operator can perform. That is a large part of why mercury carried so much weight in Chinese, Islamicate and Latin alchemy alike.
Its practical uses were real and consequential. Amalgamation gilding dissolves gold in mercury, spreads the paste on a surface and drives the mercury off with heat, and it was the standard way of gilding bronze and silver from antiquity into the nineteenth century. The same affinity made mercury the standard means of recovering gold and silver from crushed ore. Both processes poisoned the people who carried them out, and the gilders of Europe and the mercury miners of Almaden and Huancavelica paid for the metal's usefulness.
In Chinese alchemy cinnabar and mercury sit at the centre of waidan, the external elixir tradition, and several Tang emperors are recorded as having died of preparations they took. The internal tradition, neidan, later reused the same vocabulary for processes described within the body.
In the Arabic and then the Latin west, the sulphur and mercury theory made these two substances the parents of every metal, mercury supplying fluidity and body, sulphur supplying colour and combustibility. That scheme organised metallurgical and chemical thinking for the better part of a thousand years, and it is the frame within which almost every European transmutation recipe was written.
Sources The Secrets of AlchemyLawrence M. Principe with The Taoist BodyKristofer Schipper (translated by Karen C. Duval)