You can think of comparable corpora as the high-tech equivalent of what many translators call "parallel texts": original reference materials in the source and target languages, but on file rather than on paper.

Like "parallel texts", comparable corpora are made up of texts that you trust, and that are content-rich. Typically, you need to consult these materials when in doubt about the meaning or use of a term or expression, or when looking for an idiomatic turn of phrase (e.g. when you are translating into a second language). Nowadays many translators (also) use the Web for these purposes.