For me there's really just one thing. I'm learning a tonal language, but during roughly my whole first <em>year and a half </em>of study, I couldn't figure out how to actually practice the tones. Most dictionaries for my target language don't include audio, and even if I could find audio clips for some words, putting audio onto my flashcards would have been a huge hassle. I learned what the correct tone <em>was</em> for each word, but I often wasn't quite sure how it should actually <em>sound</em>, in practice.
It was only about half a year ago that I suddenly realized I could use the text-to-speech feature on my phone to hear the pronunciation of each word. In fact, it turns out that the flashcard program I've been using even has a feature in the "advanced" settings that lets you set up the cards to <em>automatically</em> read one or both sides of your flashcards out loud, in whatever language(s) you choose.
I think my pronunciation and speaking fluency would be vastly better if I'd known about this feature from the beginning! It's not a substitute for having a teacher give you personalized pronunciation feedback, but for everyday practice, it's much better than trying to learn new vocabulary without hearing it at all.