When you first visit a web site then you browser stores some of its information’s like sites logo, images, and some contents into your hard disk so that when next time you visit that site again then contents can be loaded from your disk not from web site, it saves time and bandwidth too.
Cache can be irritating sometime as you might get older contents, see no changes on websites, this is caused because contents are loaded from your disk not from web servers.
You can always hit Ctrl+F5 to force web browser to load contents directly from web server and not from cache, but it cant be used every time.
This extension is more useful for web developer and designer as they might want to see contents or changes which they had made and check whether their changes have propagated properly or not.
You can always use Private mode in which no cache is loaded from disk and web site will be loaded directly from server but it also delete cookies and other information’s where Cache Killer only clear cache not other information’s.