The Fed has a dual mandate - price stability and reducing unemployment. The second mandate was from the Humphrey Hawkins bill of 1978 ( 1977? one of those).
It's messier than that. In parts of the economy, a dollar still buys the same, but in other parts, no. you/I mainly see that because of oil shocks and either entitlements or commodities that are under subsidy that are indexed to inflation - and some other stuff.
The "other stuff" includes real estate, education, medical care and government. Real estate now, not so much. Each of these things has regulated supply ( not necessarily government regulated ) and rising demand because of GDP growth and population increase.
A great deal of stuff gets cheaper year by year, when measured by labor needed to buy something. Even subsidized commodities get cheaper. Even automobiles have, adjusted for improvements, stayed a lot the same for the last 20ish years in pure dollar terms. There's been like 1 or
2% annual increase in price, even though the offerings are usually better ( have air bags, ABS, better engine control , other stuff ).It is part of the job of the Fed. Inadequate money supply causes deflation - the overall goods and services go up due to GDP growth, population goes up. The money supply has to adjust to accommodate that growth.
Inflation is painful; but deflation can threaten the basic fabric of society. If the value of a dollar *goes up* ( which would happen even if the amount of currency,credit & specie were merely fixed ) then people are incented to hold currency rather than let it at interest, and economic activity plummets.
The question is how to regulate and govern the money supply. It is not clear that economists know how to do this; indeed, it's nearly clear that they don't . There are at least two camps and neither of the two main ones seems to work.
Our central banks are made up of ... bankers, and they have biases.