Gold is rare for a reason that has nothing to do with the Earth. Elements heavier than iron cannot be made by ordinary stellar fusion, because fusing them consumes energy rather than releasing it. They are built by neutron capture in rare and violent events: supernovae, and above all the collision of neutron stars. Observation of the neutron star merger detected by its gravitational waves in August 2017 showed the spectroscopic signature of heavy element production directly. Gold is rare because the events that make it are rare.
It is rarer still at the surface than in the planet as a whole, because gold is dense and dissolves readily in molten iron, so most of the Earth's gold sank into the core during formation. The gold that can be mined is generally thought to have arrived later, in the bombardment that followed.
Crustal abundance is around four parts per billion. The total quantity ever mined is roughly 200,000 tonnes, which would form a cube about twenty-two metres on a side. Most of it still exists, because gold is not consumed.
Its uses divide sharply. Jewellery and investment take the great majority. Industrially, gold's value is that it does not corrode and conducts well, which makes it the standard plating for electrical contacts and connectors wherever reliability matters absolutely; there is gold in every phone, every computer and every spacecraft. Gold leaf, beaten to about a ten-thousandth of a millimetre, coats domes and manuscripts and the visors of spacesuits, where it reflects infrared. Colloidal gold, particles a few nanometres across, is red rather than yellow, was used to colour medieval glass, and is now a standard reagent in medical testing: the coloured line on a lateral flow test is gold particles.