This is handy for experimenting with an idea, and potentially useful for beginners as well.
Some may notice the similarity to jsFiddle, a popular site that does essentially the same thing, though in a different way. Scratchpad greatly simplifies the UI, and has a single code pane rather than individual ones for HTML, CSS and JavaSceipt, making it a lot more newbie-friendly. Scratchpad is also using the fantastic Ace editor for the editing pane, which does a great job of feeling like a native text editor in your browser.
One really cool feature is live-saving. Each document gets a unique URL that you can copy and send to somebody, and they’ll see the same document. Changes are saved as you work, and if one user makes a change, it’s instantly mirrored on the other user’s computer. (Try it: copy the URL into another window or tab and change the CSS. It changes in the other window right away.) This could be useful to help somebody out. They could paste in a page they’re having trouble with, and you could make corrections and demonstrate the effects in real time.
Bonus tip: If you click the menu icon in the top left, you can see a list of documents you edited recently.