Why might the US raise its interest rate?

On Wednesday this week, the US Federal Reserve is widely and globally expected to raise its interest rate by one quarter of a percentage point, in its first increase since June 2006. The Economist ran an interesting leader last week on why the Fed might raise rates, effectively tightening monetary policy, taking the view that it was purely a "credibility" exercise because they had previously said that they would raise rates, rather than an example of being able to fine-tine the economy.