Because the user in question does not know and is unwilling to learn how to design anything if it is not web technology.
That is, you probably want to design PDF output. Some users prefer to do any design work whatsoever using web technology. Therefore, PDF needs to be generated from HTML.
That is, you probably want to design PDF output. Some users prefer to do any design work whatsoever using web technology. Therefore, PDF needs to be generated from HTML.