It is clinically important if ST segment is elevated or depressed as it can be a sign of ischemia (diminished blood/oxygen supply) and hyperkalemia (excessive potassium).
The ST segment is important because it can show whether ischemia or infarct is present. In general, an ST segment depression (greater than 1mm) indicates ischemia while elevation (greater than 1mm) generally indicates infarction. This is applicable to an EKG with 12 leads. The holter has 5-7 leads and that may distinquish a difference in a diagnosis?!
The holter tracings show fast heart rates, skipped beats, delayed beats, normal beats, etc. within a time frame. I'm not able to give any information on that aspect.