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It's important for the US dollar to maintain reasonably constant and predictable purchasing power from year to year. This is part of their dual mandate. Their target is just under 2% inflation per year, as measured by the Core-PCE index. If the dollar didn't have reasonably constant and predictable purchasing power, people would be afraid either to issue or to take out loans denominated in USD.

Manipulating the supply of the dollar via interest rate adjustments, along with open market operations, is how the Fed attempts to control inflation.