
Imagine this: You meet a french person, and you try to speak to them with your mediocre school french. They will tell you words you don't know yet and also what they mean, but the next day you forgot them alrady.
Or you read a book in English and you come across an unknown word: you look it up, the next day you don't remember it anymore.
Improving your vocabulary is tedious! You have look up the words, create flashcards and learn them. This is where Wortschatz makes your life easier. When you encounter a word you want to learn, you can add it. Using active recall and spaced repetition (like in Anki) you will actually be able to remember the vocabulary that you need.