An example of this could be useful if your company is a competitor to Shopify. You can quickly get a list of the websites using Shopify
for potential lead generation. You can export the list as well as set up real-time alerts as the spider encounters new sites going forward.
Another use case would be if you offered a commercial product that gets installed on websites. You could get a list of the websites using your product and then compare that list against your licensed customers. For example, sites using vBulletin