Unlike fiat money, gold cannot be manufactured cheaply.
This limits government's power to obtain it.
Think of Zimbabwe. If you were a wage earner in Zimbabwe,
would you rather keep your retirement fund in Zimbabwean
dollars or gold? These are two extremes along a continuum
of ways to store wealth.
Gold is a good way to store wealth because it is
durable, of limited supply, dense in space and value,
portable, fungible, dividable, identifiable, and
universally valued now and for the foreseeable future.
Zimbabwean paper money is portable, fungible, dividable,
and identifiable, but its performance on the other
characteristics makes it it an overall poor way to store
value.
All forms of government-created fiat money are in danger of
going the way of Zimbabwean dollar. The issuing government
can just decide to print more, thus devaluing the existing
money.
Other ways of storing value have their own problems. Some people use land, but while land is potentially very
valuable, it's not portable, fungible, or universally
valuable. Businesses organized as corporations can be used
to store value, and many people save for their retirement
this way (by buying stocks). This has the benefit that in
good times, the value of your investment can grow by
itself. Unfortunately, it can also shrink by itself in bad
times. Also, shares of corporations are not easily dividable.
My point is that forms of money (stored wealth) are not
arbitrary.