There are many benefits to letting very young children draw, scribble, and write. You'll find some great articles detailing the many benefits of letting children express themselves on paper toward the end of this post.
Here are some examples from my own children when they were young.
Luke created this when he was four years old. He called it his book. He folded it in half and would open it and "read it". He was very proud of his book and brought it out often.
Hailey created this picture when she was three. She enjoyed making marks on the paper and using different colors.
Eric finger painted all the animals from a book. Can you see all the animals from Brown Bear, Brown Bear, What Do You See? Here's a video of my son pretend reading this book when he was three years old.
Here are some articles written on this subject.