One of the qualities of a good writer is conciseness - not using two words when one will do. In the words of a famous linguist, writing is half expression (putting ideas into words) and half suppression (doing without words). Redundant words are especially irritating because they divert attention from your main point. Fortunately, English provides a number of useful tools for omitting unnecessary words.
One of the tools is the little word "the", which in traditional grammar had a special name all to itself: the definite article. This is always combined with a noun, and (as usual) the noun "refers to" some person or thing. In grammar-speak, this is the noun's "referent"-the particular person or thing that it picks out, which of course varies from moment to moment. The referent is very different from the dictionary meaning, which stays (more or less) the same. For example, every time you use the word "boy", it has the same dictionary meaning, but its referent is Tom on one occasion, Dick on another and Harry on a third.
The effect of combining this little word with a noun is to give a very important piece of information about the noun: the reader already knows its referent. For instance, the referent of "the boy" isn't any old boy, but one of the boys that the reader already knows; and moreover, it's one that the reader ought to be able to pick out immediately. Often this will be a boy who has just been mentioned. For example: "On my way to school, I saw a boy stroking a dog. The dog seemed to know the boy ..." In other cases the boy will be identifiable in some other way, as in: "The boy is late again", referring to the boy who delivers the papers. But in every case, "the" triggers a search in memory for the intended referent. (In contrast, the indefinite article "a" is an instruction to introduce someone we haven't already met.) Any native speaker is already an expert user of "the" in everyday conversation, but some novice writers seem to think that it's cheating in writing. Writing is all about being explicit, they feel. They clearly haven't been told about suppression. For example, one imaginative KS3 writer described a lonely man who spent all his time painting, so that his flat filled with "the canvases that he painted". Those three words "that he painted" are pure distraction for us readers, because "the canvases" has already taken us to the intended referent. Worse still, the extra words start a wild-goose chase for some other, previously unmentioned, canvases.
Maybe KS3 writers don't really believe that a little word like "the" can do so much work, so they pile on a few real words just to make sure. One important lesson in becoming a better writer is it depends as much on the words you don't use as on those that you do.