Many translations are about a subject that often has a lot of background information attached to it. Museum annotations, information leaflets, historical documents, all have deep meanings and knowledge behind them. So gaining even a little enlightenment about the details can have a big impact on the quality of your translation.
Understanding just some of the background knowledge makes translating easier too. The process often gives hints on how the translation should read and feel, which can be fed into the style of the text/s
Overall, the more time that is invested in the translation, the better it will be. So the more research you invest in the background material the better. Of course there are often time constraints for translators who work on many documents, so getting the balance between research and writing can be tricky at first.
A greater understanding of the subject matter allows for a more informed and interesting piece of text.