WordPress HTML allows you to add custom HTML to both the post/page BODY and the HEAD tags from the page editor.
When copying HTML into the WordPress editor it adds spurious tags which break various elements and corrupt the HTML. By saving the HTML in the custom fields dialogue the exact HTML will be output to your post/page.
Also if you have a single page which requires an extra javascript library or style sheet you normally have to add it through your themes php files. Updating the theme files can be a pain; we normally have to set up an exception for that individual page and load it there. As this information is not available when we are editing the actual pages in the WordPress editor it is quite obstructive.
With WordPress HTML, we can add the library or stylesheet directly to the post/page head without the need to change the theme files. And importantly, all from within the WordPress editor for that page.
Example libraries: jQuery, ChartJS, D3js, Highcharts, Prototype
More details: WordPress HTML
Code: GitHub