It's an initiation into the SE format. If you spend time asking and answering questions, you will see other people's comments. You will learn what is and isn't acceptable in a comment, before you have the opportunity to post one. As the comment box says:
Use comments to ask for clarification or add more information. Avoid comments like "+1" or "thanks".
In addition, comments are not for asking related questions; if you have a different question, you're supposed to ask it.
Both of these rules are integral parts of the SE format, and crucially not how forums work. Most people who are new to the SE format have forum experience, and expect SE to work like a forum. By requiring users to participate initially through only questions and answers, we give them time to get used to our idea of what is a valid question, what is a valid answer, and what is an acceptable comment. Then by the time they have contributed positively to the site and earned enough rep to comment, they will hopefully comment responsibly.