The "advantage" was probably one of not exceeding the original author's capabilities. It might have been a case of "you do what you can with what you have". Getting a simple page working, and then copy the HTML into a CGI is a pretty simple way to get started, though longer term maintenance is a hassle.

Time passes. The original author gets more competent, or a new, more competent developer (you) comes along.

It's now a business decision as to whether investing time to clean this code up, say, by separating the HTML out into templates, makes sense. Your task is to provide management with information about the cost of the status quo (high on-going maintenance) versus the cost of a rewrite (some cost now, lower ongoing maintenance).

They may weigh the risks and decide on status quo.