Days of cover expresses inventory as the number of days of demand it would satisfy. It is inventory divided by a daily consumption rate, and it exists because the raw stock level answers the wrong question.
The reason to bother is that both the numerator and the world grow. A stock level that was comfortable when demand was seventy million barrels a day is a different thing when demand is a hundred million. Comparing today's inventory to a level from twenty years ago without normalising compares a buffer against a much larger flow, and concludes that stocks are high when the cover may be unchanged or lower.
The same argument applies across regions. Two countries holding identical volumes have very different security positions if one consumes twice as much as the other. Cover is the comparable quantity; volume is not.
The metric is also used in a regulatory sense that is worth not confusing with the analytical one. IEA member countries hold an obligation to maintain emergency reserves equivalent to ninety days of net imports, which is a rule about a specific definition of cover, computed on net imports rather than on consumption. A country can be comfortable on one definition and short on the other.