Why is the case of U.S. v. Booker so important to federal sentencing?

In 2005, the U.S. Supreme Court decided the case of United States v. Booker, which held that although the U.S. Sentencing Guidelines–in effect since 1987 in the federal courts– may continue to be used by judges in fashioning an appropriate sentence, they may not be used in a mandatory manner as they were prior to Booker.

That is to say, while the sentencing judge should continue to consult the guidelines in arriving at an appropriate sentence, they may depart from the guidelines when it is necessary to do so in order to impose a reasonable sentence utilizing the various factors set forth in Title 18 section 3553 of the United State Code.