On the oft stated premise that Volume precedes price, Volume Indicators try to examine volume action as a prelude to price action, often on both sides, both as a continuation signal and/or as a reversal signal.
As is usually the case with all market indicators, the story that volume tells is often one that needs to be seen in the context of the broader market action. While extremes of volume often tell a story, just what that story is is more a matter of interpretation. There is no Holy Grail.