1. Readers understand words
Readers understand words because they have read how sentences and words fit together, the order of a sentence and the placement of words. They understand the sentences because they have seen these words and similar sentences before. They can then use them in writing or compose similar ones.
2. Readers understand the structure of the Language
Readers have seen the structure so many times that they can readily understand how to construct great sentences. They can see that writers use different kinds of sentences to make Writing more interesting like starting out with a question. Then the author proceeds to answer it. A variety of sentences like Declarative, Interrogative, Imperative and Exclamatory make Writing much more delightful to read. Readers also see that some sentences are long and some are shorter. By varying sentence length, it keeps the Writing more lively.
3. Readers understand grammar
Good grammar only makes sense to readers. They know how different parts of speech should fit together. For example: they know in the sentence "The boy rides his bike," that rides goes with a single subject noun. Thus, they understand subject-verb agreement. Understanding good grammar really can assist the reader in Writing. They are much more able to write complex and simple sentences.
4. Readers understand thoughts of the writer
Good readers understand the thoughts of the writer because they have read and thought along with the authors. This helps them in Writing their own compositions. This is a tremendous asset in Writing. When you read, you follow the author's train of thought. It is so natural.
5. Readers have a great vocabulary
Because of their tremendous reading, readers have a great vocabulary. This will certainly help in Writing. They will use a variety of words, not the same old thing. I have a cousin who was such a wonderful reader. When he wrote his thesis for his doctorate, the words he used were so large and long, one needed a dictionary to look them up. He always beat everyone in Scrabble when he was little, even the adults. No one could outdo him. His vocabulary was extensive.
6. Readers know how to make their writing interesting and exciting
Readers read. They know they do not like to read "boring" books or articles. They can see how authors use words to paint pictures and make it more interesting and exciting. Redundancy is poor Writing and writers need to stay away from it. Usually good readers are good writers. They go hand in hand. One complements the other.
7. Readers can use creative ways to make people want to read what they wrote
Finally, I would say that good readers can make their Writing creative. People want to read it. It would be a book you can not put down. You just want to keep reading it until the end. It is like a bait to a fish. I thought The Shack was like that. It had a great story line, the kind that would interest many people. A good reader can do that. I loved it when my students were creative in Writing. Sometimes I would put a sentence or part of a sentence on the board. It was great to see their creativity using that sentence and making an adventurous story from it.