I've found spaced repetition a great way to increase the size of my vocabulary whilst learning a language. You're right that it's unnatural, but a large unnaturally-learned vocabulary can then be polished by practice in more natural contents, and I feel like the net result is more efficient than using the natural contexts alone. This is especially true since I do not have easy access to native speakers for half an hour every day, but I can practice with the Anki app whilst on public transport during that time.

Last time I learned a language (German), a small vocabulary was my perpetual weakness. Now I'm learning a different language (Spanish) with a different approach that makes heavy use of spaced repetition, and vocabulary is no longer my weakest point. Instead, listening comprehension is now my weakness. But with a decent vocabulary, I am now working on improving the listening comprehension using podcasts. If my vocabulary were weaker, this would be much more difficult as I would need access to a native speaker who would considerately restrict themselves to mostly use vocabulary I know.