By turning the knobs you offset the start and end positions, so that they will play back more or less of a samples' waveform. Typical examples of use for this would be:
Removing unwanted portions from samples.
This could be anything from noise to "dead air" at the beginning or end of a sample.
To create variations out of a single sample.
These controls can be used to pick out any section of a recording for use as a sample.
Together with velocity sample start control.
You can for example increase Sample Start and then apply negative velocity modulation to Sample Start. Then, the harder you play the more you will hear of the attack portion of the sound.
If you hold down [Shift] when adjusting these parameters, the adjustment is always in single frames (samples).