Why are you only allowed to comment on other people's posts when you reached 50 reputation? In my opinion comments are a good way to ask people to clarify their questions or add additional information to problems. That's why I see no sense in limiting people's ability to comment questions. But maybe I'm just overlooking something here.
What are the reasons for that policy?
Take a look at this answer from one of the creators of the software Ask Ubuntu runs on.
There may be people on Ask Ubuntu who disagree with this, but we're unlikely to see it change.
There's a 50 rep threshold on comments mainly because we want users to focus first on the core part of a Q&A site -- asking great questions and providing great answers!
Also, comments tend to be meta-commentary, which while useful and often illuminating, isn't providing an answer.
We want users to learn this distinction before they are allowed to comment, hence, the requirement to earn 50 reputation.
(and remember, you can always comment on your own posts (and any answers to your questions) with only 1 reputation.)
For a new user (who might not be familiar with reputation), it can be incredibly frustrating to see comments but see no UI for commenting. The UI just looks wrong, broken, and inconsistent.. The "answer" being the only way one can say something, new users will just use that if they really want to say something. For example, this very answer!
It is quite reasonable to give more information about a question (even someone else's question) without answering it - eg, someone may see an "I have XYZ problem, how do I solve it?" question, and they might have the same or similar problem: they might be able to contribute by describing the problem in more detail, or by giving their insight on why the problem occurs, even if they can't actually solve it.
