Sampling rate is a most important parameter that determine audio quality. The sampling rate is the number of samples of a sound that are taken per second to represent the audio. The more samples taken per second, the more accurate the digital representation of the sound can be.
All audio formats and audio hardware support only limited range of sampling rates. Here is a few cases when you need to convert sampling rate:
Audio Converter Plus will take care about sampling rate conversion automatically, but you can force it to use any frequency in codec settings.