They know the law. Laws change and in the last 3 years there have been several key law changes affecting what happens at the end of a fixed tenancy, what notice you have to give to end a tenancy and the length of time you have to comply with Healthy Homes requirements to name a few. Unless you are spending time at seminars and webinars its best to leave it to the professionals.
They will deal with any disputes. Even nice tenants can turn nasty when it comes to moving out. They aren’t always leaving a property under ideal circumstances (a job loss or split in a relationship) and can retaliate when it comes to how clean the oven has to be. There is no need to deal direct with the tenants.
They can ultimately earn you a better return because they know the market rent and they know the tenants. They can negotiate fair and regular rent increases which many private landlords find uncomfortable and difficult to do.