No matter how proficient you are, when you read a book, you employ your memory! There are characters, sentences, and events (if you're reading a fiction book) that you need to remember for maximum fluency.
When you read, all of your focus is on the book. If you're doing a read-aloud, this is even more true. Over time, all this practice of focusing adds up. All you need to do is read a couple of pages a day to develop a habit.
Another perk of reading is that you'll gain new knowledge of words! You know that quirky friend/coworker you have that's always showing you up with new fancy vocabulary… you can do an uno-reverse on them one day if you read enough!
When you read, not only will your comprehension skills go up, but so will your writing ability. Exposure to great pieces of literature has an effect on your writing abilities beyond just vocabulary words.