HTML is a file format that can hardly be presented in a way that guarantees it to be read on any device, which is something that the PDF format actually does. Therefore, converting HTML to PDF will let you transform the website’s URL you want, or your file, into a document that can be read from all operating systems.
A website, for example, can be downloaded in HTML format to contain all its construction in the same document. But then, if you want to convert that HTML into a PDF, you can use an online converter, which facilitates the task without the need for programs. You can do it both through the website’s URL and through the previously downloaded .html file.
Of course, you can always reverse the action of converting a PDF to HTML by performing the same steps the other way around.