Custom Layers - How it works on unity Sentis? Whats the purpose it aims to achieve?

I have an onnx model for an application that we have built using Unity’s Barracuda. Its an object detection model. I am planning to convert it to now work on Sentis. I came across the upgrades sentis is bringing in to the table. I was curious how custom layers are built in c# unity and can you give an example on where it can be mostly used? A real time example would be helpful for me and devs like me to understand and utilize Sentis to the fullest.

com.unity.sentis\Samples\Add a custom layer
The point is that is there is a layer that we do not support, you can implement it yourself.
You can also also provide your own optimized implementation (either for layer fusing or overriding our implementations)