I recently switched to HTML based payment terms on my invoice. As I’m not very skilled in HTML code I needed to repeatedly check my code to see if it produced the intended result. The only way I could do this was to create a draft invoice, write the HTML code there, then use the preview option to check the results.
Once I had achieved the desired result I then pasted my code in the invoice template. It would be far more convenient (and logical) to create the code directly in the template and preview the results there, instead of having to create a live, draft invoice.
The benefit of having a preview option is, of course, not limited to HTML code only, or indeed invoices.