What For?
All the years I've studied CHinese with physical flashcards, I always used them both ways - Chinese-English, and ENglish-Chinese. Once I knew the characters and pinyin, I'd jumble them up and flip them over. So part of it is just another way of randomizing the associations, and strengthening the learning.
As an entirely separate matter, I use them because I still occassionally operate in English, and want to have an automatic recall of the Chinese terms for common English phrases - this is particularly common for technical and financial terms.
Finally, in the same way that a single Chinese phrase or character can have many meanings, a single English phrase or word can have several possible Chinese meanings, depending on context. It's often useful to have all possible chinese meanings tied to one english word or phrase. Same reason you have an english-chinese dictionary, of course : but the whole reason for a flashcard is to simplify memorization of certain dictionary entries, and that need is as strong in one direction as another.