To explain cached data simply, cache is a collection of images and various file types being stored on your device by websites or applications you make use of or browse. That is great and all but what does this mean for you as the user making use of the website or application?
In a nutshell because of cached data the files on your device are already available when you choose to browse that particular website again which reduces latency of the page being browsed. This is because your device has already stored the files from your previous session, making loading time faster and this results in quicker browsing for the user.
