If you have a gold standard where dollars are redeemable in gold, then the threat of people trading in all of their dollars for gold is usually enough incentive to prevent the government/central bank from creating too much money.
If you don't have a gold standard but you allow competing currencies, if the government/central bank inflates the money supply too much the people will just refuse to accept dollars and will prefer payment in gold or silver, thus providing similar incentive to being on a gold standard.
The reason for both of these is to restrict the ability of the central bank to print too much money, debasing and devaluing the currency. If you need to know why inflating the money supply is a bad thing, there are several threads in the Economics section of this site that touch on that topic.