1. Adding a Banner or Pop-up to a Page
One way to utilize custom HTML options is through adding a banner on to a page. Let's say you have a deals page and are looking to increase bookings or draw attention to a specific deal. One way to do this would be with a pop-up or a banner on the page. 
This can be easily accomplished with a little knowledge in code and some time. First go into the page you want to edit, in this case the deals page. Then navigate to Advanced Settings in the page configuration. Once there you'll see custom HTML. Click edit to open the code editor and write your code. Save your code and there you have it!
2. Adding a Tracking Script to a Page
This HTML block area can also be a super helpful way to add a <script> tag to your site. If you work with any external tracking or software that asks you to incorporate a <script> tag on to your page you can do so with the HTML editor.
3. Page Modifications
One other way to use this feature is for one-off page modifications. Say you want to change the font on one specific element, well if you inspect that element and find the class you can then write custom styles for that class in a <style> tag.
And there you go, you know now how to incorporate custom HTML into your page for new advanced modifications.
Happy coding!