"~는 데" 데 is similar to 곳(place), 것(thing), or 일(matter), but can also has a more abstract sense like "a case", "utility", or "purpose". - 돈을 바꾸는 데[곳]를 환전소라고 한다. (곳 is better, but 데 can be used too). - 그는 이기는 데[것]에만 관심이 있다 (He is interested in winning only) - 그 책을 다 읽는 데[일에] 삼 일이 걸렸다.(It took three days to finish reading the book) - 특히 영어를 더 자연스럽게 말하는 데[것, 일]에 있어서요. (Especially for the purpose of speaking English more naturally) - 실력(을) 늘리는 데[것에, 목적에] 도움이 돼요. (It helps in improving your skills) * 늘리다 (not 늘라다): expand, augment, improve. So ~는 데 not followed by any particle can be taken as a contraction of ~는 데에, and usually means "in the case of", or "for the purpose/utility/application of". ~는 데 is sometimes confused with ~ㄴ데 (no space) which is a verb connective form with the sense of "when", "as", or "even if" ("눈이 오는데 춥지는 않다"). The two are unrelated.

Understand this as a kind of universal preposition. It can be many different ones depending on the context. 영어공부 하는데 도움이 되다. To be of help for studying English. 돈을 버는데 집착하다 To be obssessed with earning money. 기부하는데 인색하다 To be miserly at(?) donating. 특히 영어를 더 자연스럽게 말하는데 있어서요... Especially for the sake of (apropos) speaking English more naturally....