Learning To Read A Language
When learning a language, you will notice that words don't always translate directly. Especially collocations are often translated using entirely different words in meaning. When you have learned individual words and encounter such a sentence, you will not know what to make of it. That is why it is important to learn words while reading. This is the first step to building a useful vocabulary with which you can build sentences.
When learning words through flash cards, you end up storing individual words in your brain which is very unnatural. Even when reading in the usual language course, this often comes down to one two hundred word text per chapter about someone going to a hotel or a restaurant. Although the words learned might be applicable outside these texts, you will not understand a new sentence if you encounter the words outside of their earlier context. Mostly you will learn individual words or words in short sentences. Read more about why it is impossible to learn words out of context.
Reading To Learn A Language
The above picture shows the difference between learning individual words or even sentences and words as part of a story. The latter is the more natural way for your brain to acquire a new language. When reading actual books, you're reading ten thousands of words, and you will absorb thousands of sentences and every time you encounter a word you met before, its retention becomes more natural and part of a organic language scheme in your head.
Collocations can never be learned outside the context of a story and most words need to be encountered in different settings or sentences to be able to understand their full meaning. That is why reading is the best way to learn a language. Another important thing about learning words via stories is that because of the volume of vocabulary reading offers you will encounter headwords and roots in many more forms than in any other format, and you will consequently learn to recognize roots better, which in turn supports independent reading. If you know 1,000 roots AND you can recognize them you know 100,000 words based on these roots. This process of learning to recognize roots is something that is automatically picked up by your brain when reading.