In the modern world people don't buy gold to make transactions but to store wealth. No wealthy person would store their fortune in fiat (paper) currencies such as dollars, but in things with real value such as shares in a companies, property, or other things of value such as gold or silver. At any time these floating currencies can change their value (think hyperinflation, or just exchange rates).

Compared to other "real" things, gold can be liquidated into dollars VERY quickly if you need to make a transaction, and since it's a real thing, it cant disappear.

Also, often gold is stored by the company you bought it from on your behalf, so you can sell it back instantly online. So no need to have to have a home safe.

Also, at an average return of 10%/year over the past 10 years (at least in australian dollars) it's has been out performing many other investment options.