Importance is part of the Collection Risk Assessment Framework.
In an attempt to graph how important an item might be, we can take into account the Consequence of Loss of that content, if we don’t attempt to preserve it, and graph against the Urgency of the item. This gives us the concept of Importance.
These tools can assist in the assessment of a large collection of materials and assist you to decide at one end of the spectrum; those items you really do need to preserve and value and at the other end those items that are outside your collection guidelines, that could possibly be de-accessioned from your collection.
You can identify high value, at risk material to assist in targeting your preservation resources; distinguishing it from material that is at high risk but of low value;
Of course, you are may well find a large amount of material in the middle!