The centre hole acts as a hose guide to protect your hose. There are 5 breather holes so you can still hear what is going on down in the drain without getting covered in effluent.
The plate is made from recycled rubber and is heavy enough to prevent the nozzle from coming back out from the drain. The centring disc is secured with stainless bolts for long service life.
It is also useful to help guide your camera into a drain to protect the pushrod from sharp pipe edges.