Why does the US Federal Reserve raise interest rates?
One of the primary roles of the US Federal Reserve is to control the supply of money within the economy.  During times when the economy is doing incredibly well, a.k.a. running hot or facing inflation (rise of prices of goods and services), the Federal Reserve will need to reduce the supply of money by increasing the cost of borrowing.  

The primary lever to increase the cost of borrowing is by increasing the Federal Funds rate (the rate for banks to borrow and lend their excess reserves to meet liquidity requirements). Increasing the Federal Funds rate results in more expensive borrowing costs, which can reduce demand among banks to borrow money.  Less money is borrowed to help fund the economy resulting in a reduction in purchasing power and the money supply.  This will slow down the pace of economic growth and help ease inflation.
Por que o Federal Reserve dos EUA aumenta as taxas de juros?
Uma das funções primárias da Reserva Federal dos EUA é controlar a oferta de dinheiro dentro da economia. Durante os tempos em que a economia está indo incrivelmente bem, ou seja, em excesso de calor ou enfrentando inflação (aumento dos preços de bens e serviços), a Reserva Federal precisará reduzir a oferta de dinheiro aumentando o custo do empréstimo. A principal alavanca para aumentar o custo do empréstimo é aumentando a taxa de fundos federais (a taxa para os bancos emprestarem e emprestarem suas reservas excessivas para atender aos requisitos de liquidez). Aumentar a taxa de fundos federais resulta em custos de empréstimo mais caros, o que pode reduzir a demanda entre os bancos para emprestar dinheiro.