Your answer can quote a statement or excerpt from another source or author, but it's a good idea to explain why you're quoting that excerpt or statement, and you should make it clear that you're quoting something, otherwise, it could be considered plagiarism, which is at least discouraged, and your answer can be deleted (as explained here).
To quote something, make sure to precede it with
> (explained here), and provide the name of the source/author (and ideally also the link to it, especially if freely available). This is explained well here, but I think you're already familiar with this.
An example of my answers where I thought that quoting would have been a good idea is this one, but note that I didn't just quote the book, but I also tried to answer the question specifically based on my interpretation of that quote and knowledge, and I also tried to explain why I am quoting the book. In this case, I was also more or less familiar with the topic, so I am relatively sure that my interpretation of the excerpt (but not just that, because that specific answer was also based on my knowledge of the topic) and specific answer are correct.
A quote-only answer may also be discouraged because I think we want to be a source of original content (which doesn't necessarily mean "original science", otherwise, you may just try publishing your new findings on a journal or conference proceedings; in any case, note that, in most cases, I came up with answers and solutions that I didn't find elsewhere; sometimes, you can find the partial answer to your question in a book or on the web, but not the full answer; for example, in this case, I really tried to make the full derivation, but, without my previous knowledge of the topic, probably, I would not have been able to do it). I am not sure if this is a policy, but I think it's pretty clear that, if we all just copied from other sources, there would be no reason for us to exist (you can just search for your answer elsewhere).