I think this site will be better if new users are able to comment questions or answers, rather than having to post comments as answers.
I found this post, but my question is why does a user need 50 reputation to post a comment?
The reason is avoiding comments are used to spam.
Spammers don't write post questions, or answer questions before spamming. As consequence, they cannot write comments (since their reputation is 1), which would not cause the question to be bumped on the front page. This means only few users would notice somebody wrote a spam comment: the users who wrote the post for which the comment has been written. Those users are always notified of new comments for the post they created.
Since spammers cannot create comments, they can only use questions, or answers to spam. Those are immediately visible in the front page, where everybody can and flag them.
The alternative would be making the comments bump a question in the front page; this is something undesirable.
Comments fill a very specific purpose which is not immediately apparent until you have a bit of experience the site. The minimal reputation requirement is a way to show you have that experience before you use that feature.
Without that experience, most incoming users would likely jump to using comments like miniature forums and chat rooms… which they are not. It's the same philosophy of why we delay voting abilities, editing, closing, etc — simply asking you to hold off using those features until you have a bit more experience with the site.
I turned up this in my search, but it was a bit tricky to find - what's the justification for the commenting reputation requirement?.
The basic reasoning seems to be as follows (taken from the linked question) - but I'm sure a mod can expand further on this :):
The justification is simple: we don't want the site to be flooded by comments from passers-by.
50 rep points is not an extremely high threshold. You can bookmark the question (not necessarily in your browser, just use the "favorite" star), gain some rep, and then return to it later.