What are the issues with quote only answer?

Suppose I found a question that I can answer. Since I am not good in phrasing statements in English, I can answer it using only the quote from a credible source.

Why such answers are discouraged?

If no, can I just provide the reference and the quote to answer?

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.