Answers shouldn’t be posted as comments, and is actually discouraged on the commenting privilege page:
Answering a question or providing an alternate solution to an existing answer; instead, post an actual answer (or edit to expand an existing one);
Comments lack quality control, are harder to find than answers (making them a lot more likely to be missed), can be deleted at anytime without warning, and don’t have as robust formating as answers (i.e. no images, shorter, no paragraphs) which make them harder to read than answers as well. If you see a low-rep user posting an answer as a comment then feel free to encourage them to post it as answer, since they may not fully understand the difference or realize all the benefits that go along with posting an answer. If they’re a higher rep user then they likely already know all this, and decided not to post an answer for whatever reason, so I wouldn’t bother commenting at all.
However, every comment which contains helpful information isn’t an answer, and they shouldn’t be treated as such. There’s a bunch of reasons why users might post helpful information as a comment, but the three most common reasons I see are:
Helpful Resources: A user might not have an answer themselves, but they might know another resource (i.e. another question, a wiki) which could help the asker (either by answering the question, or providing enough information for the asker to figure it out themselves). Posting a link like this wouldn’t be acceptable as an answer, but it's totally fine as a comment.
Incomplete Answer: A user might be able to answer part of an askers question, but not all of it. A user might then leave a comment containing what they know, in the hopes they it’ll give the asker enough information to figure out the rest for themselves. Like in the above scenario you might even see the commenter follow up with an actual answer later, once they’ve had time to do some more research.This is another example of a post which wouldn’t have been acceptable as an answer, but still contains helpful information for the asker, making it a solid comment.
Uncertainty: A user might think they know the solution to an askers problem, but they might not be sure (either about certain parts or the whole thing), and so aren’t comfortable posting an answer. Instead they post a short comment, since they really want to help the asker, but then follow up with a more in-depth answer later on once they’ve done some further research. This is the only comment which could have actually been an answer, but users who post this kind of comment will usually just delete it once they’ve had time to do some research.
Each of these reasons is quite a bit different from each other, but the key thing they all have in common is that they’re posting helpful information before a proper answer has been posted. In some cases this proper answer will be posted by the commenter, but other times that helpful comment they left could actually lead to the asker or another user to figuring out a solution to the question. If that happens then an answer would be posted by whoever figured out a complete solution, which is good for everyone.
Once a complete answer has been posted the comment isn’t really useful anymore, but that’s totally fine, because comments are only meant to be tempoary ‘post-it’ notes anyways. If one of these comments has outlived it’s usefulness then it can be flagged for deletion via the obsolete flag, just like any other obsolete comment. There’s no reason to delete the comment before that, though, and we’d just be doing a disservice to the site by doing so.
So, in summary, no answers shouldn’t be posted as comments, but just because a comment contains helpful information doesn’t mean it’s an answer. When those helpful comments are posted either upvote them or ignore them, but don’t flag them, at least not until an answer has been posted which makes the comment obsolete.