Why do some countries keep their gold reserves in others?
Gold is still used to settle the balance of payments. In NYC there is the biggest gold deposit in the world, even bigger than Fort Knox. Each nation that has gold there has its own vault. When they have to pay bills, gold is moved from vault to vault. That is a lot safer and more secure than actually shipping the gold to another country.
What USAF wrote plus It's a form of insurance to keep gold deposits in a safe location like London. Keeping it at home would be way too much temptation to armed groups and revolutionaries.
Gold values varies from country to country. So your gold supply could be worth more in a foreign country and moving it will end up costing you.