The HTML block gives you the ability to paste code directly into your page.
If you've got an embedded code or some JavaScript you just need to paste on the page, this is a great way to do it. If you've got some complicated HTML you've made in another program (like a big table layout) this is probably a better block to use than the content block.
This block will never try to clean up your code or do any formatting on it. It will highlight it and suggest fixes for you as you type it, though. So it's got that going for it, which is nice.
