Lots of things.
We should probably inquire as to why people prefer it as a medium of exchange…
They don’t necessarily prefer it. Legal tender laws, domestically, virtually force citizens to pay taxes in them; the Fed Res Note is a means to pay taxes, where silver and gold would only be counted as much as the government says that they are worth. For instance, gold is currently at $1780 per ounce, but the government, I think, still only recognizes an ounce of gold as $45. So paying taxes in gold is a bad idea.
You can kind of mix legal tender laws with federal budget deficit*.* The legal tender laws ensure that the US can create Treasury debt and force a demand for them through the payment taxes.
Fiat or debt standard (pending on how you look at it), the lack of a gold standard allows banks to create as much credit as they want and for governments to borrow as much as they want. When you can make loans indefinitely, and your legal system won’t get your for fraud or counterfeiting, you can make a lot of good loans (loans that are successfully paid back. There are obvious flaws here, but bailouts make sure it still “holds together”) and keep the demand for your service high. So, in turn, a world with an excessive capacity for low interest debt can come to the phantom money factory and get it; they like it this way. But, mostly that is just banks and their immediate agents. I would also say that “excessive capacity” comes from the ability of the loan originators to make as many loans as they want (central banks).
You can mix this in with Exchange rate value, but that situation is deteriorating rapidly.
Agreements with other states (or more likely their central banks) to hold certain quantities of US dollars in the reserves (lots of countries do this). When countries hold us dollars it means that they are capitalizing their own debt based fiat currency with our debt based fiat currency.
Private institutions, banks, and international organizations also use “eurodollars” to capitalize their doings. A eurodollar is just a us dollar that is kept in an account anywhere in the world other than the US.
…and the petrodollar scheme; this method of securing demand for US paper has been very effective. People want their country to grow and ultimately that growth is predicated on energy resources. So, if a country wants to expand production and the place where they get their oil from is on the petrodollar standard, they must acquire US dollars to buy their energy resources. This is why Saudi Arabia is crucial to us energy/financial policy.
Only because “they” haven’t figured out how to move to a floating fiat basket reserve. They have theorized about an IMF based sythetic currency, but the East isn’t really down for that. My guess is that more and more countries will attempt to create their own petro-currency standards. Hopefully in doing this it will stagnate the economic power that States get in the event of securing a petro-currency standard in a foreign oil producting nation.
Qeustion 2. has been answered.