When your child is able to read books, you can fine tune their reading more easily. You can also spend more time on the basics, ensuring that they read better than even children who are older than them. This will ensure that when they get to school (if you’re not home schooling), they will be fully prepared and can have more fun and be more relaxed in their classes; and when learning is fun it is more easily retained.
When my sons were in middle school, they loved participating in the “Battle of the Books,” a nation-wide program to promote reading. Kids formed teams and read selected books that were high quality and age-appropriate. Then they competed against other teams at their school and other schools, answering questions about the books: characters, plot, symbolism, etc. The team and competition aspect motivated my boys to read, and they had a fun time doing it. I also read a couple of the books aloud to them. Because the books were chosen by a committee of professionals, I enjoyed reading them and discussing the important ideas they covered (immigration, bullying, discrimination) with my sons.