When you want to learn how to say something correctly, you can’t always look it up. To give an example, I have been using 数量for a while to talk about the quantity of anything and everything. However, as I was translating J>E I realized that the author of the document used 個数 and it dawned on me that I have seen tons of occurrences like that but never incorporated it into my own Japanese. (枚数、件数、トン数など)
If you do J>E, the original document can be one of your best study resources ever. Because you already know what you want to say in English. You can see what you translated the original into and then remember, when you see something like that come up in your E>J work, you will know what phrasing a native speaker would use.