Your employees' networks are most likely composed of people with the same background and experience, like former workmates and schoolmates.
Your employees know the company best and will only refer candidates they know fit in, resulting in higher quality candidates.
Candidates get a glimpse of the company's culture through the employee before they apply. They come in with a better understanding, resulting in better retention.
Empowering employees throughout the hiring process causes them to be more engaged in the internal operations of your company.