What is the significance of Impression share metric?

A PPC campaign takes help of various metrics to check the performance of your campaign. One important metrics among them is Impression share metric that marketers have to eye on for checking the efficiency of their ads. It is gauged by dividing the impressions you’ve received on the search network with the estimated number of impressions you were eligible to receive. High search impression share is a positive indicator of whether or not your budget can support the search volume of your keywords.