Of course they have meanings, and are very useful in sentences! 'I bought a coat/ went to a restaurant/ have a Brazilian friend' is to introduce something 'my coat/ the restaurant we went to/ my Brazilian friend' are to reference something you should already know about. also, 'take the train/play the guitar/ go to the cinema' are for general things (not thinking about which one) :)

你好，Jiali Qu 我也是英语学者。我是如此了解它。 Why the English language has articles? First of all, not all nouns need articles.I can say: I like dogs. As to your question of why, the answer is that's how English works. The articles used to distiguish between new and old information. For instance: I have a dog, the dog is a mixed breed. Here, the article “a” refers to a new information. While the article “the” refers to “my dog” that has been mentioned before (it's not the other's dog). 看看英语母语者如何看待这个疑问。

We don't really "add articles" to nouns. We use noun phrases, and many noun phrases include articles. English has definite noun phrases, which refer to identifiable referents, and indefinite noun phrases. It might help if you started to think in terms of noun phrases or noun groups, and not of articles as single words. Also, you should realize that not all languages have the same grammar.