Hey Abhay, PageFactory annotation @CacheLookup is used to mark the WebElements once located so that the same instance in the DOM can always be used. CacheLookup attribute can be used to instruct the InitElements() method to cache the element once its located and so that it will not be searched over and over again.
It is useful when the elements that are always going to be there. For AJAX based applications, it may not work where the DOM changes based on user action on the page, otherwise every time when we use a Web Element the WebDriver will go and search it again. For example:
private WebElement userName;