This insight shows you a trending line graph of the number of new customers to your storefront in the given time period. Customers that you add to your shopify account manually will also appear in this insight. So it is possible this includes customers who have not necessarily purchased from your store.
This insight helps you track trends in new customers. If you see a spike in new customers, it could be because you just added a new product to your store and had some marketing around it. Track sources that lead to new customers and focus on promoting your business in those sources.