Suppose you want to download various articles from a website, concatenate them into a little book with a table of contents, and read that book offline, on your Kindle or tablet? This program lets you do that. It will also remove any dangerous tags (such as scripting) and lets you decide what tags you want to keep.
If you see a tutorial on the web which is divided into many pages with NEXT buttons, you can also concatenate all those pages easily into a readable offline file.
If you are a programmer who has just seen a cool site, this lets you download the javascript files that power the site.