Some vibration software and vibration data collectors allow the use of frequency bands to help the analyst measure and identify specific equipment faults.  These frequency bands are actually measured in the vibration data collector.  The results can be used for trending and alarm purposes.  You can create specific bands around specific faults that may occur in the equipment being monitored.  For example, bands for imbalance, misalignment, specific bearing defects, electrical defects, and much more can easily be measured.  Alarm thresholds can be created to alert you when attention is required.  These bands can help you identify equipment issues in your vibration data before you look at the FFT or time waveform data.  A review of the FFT and time waveform data should always be completed.  However, band alarms can keep you from overlooking an issue and reduce the amount of analysis that may be required.