About custom event

I have gone through the documentation but still unable to understand the Custom Event and how it could be helpful to me.?

can someone explain?

After reading part of the documentation about Custom Event, I think it is helpful for you to organize your workflow. Like the example in the documentation (figure below), if a user sign up, you can trigger your custom event to send him/her email. If there is another button that also trigger to send email, you can reuse the custom event, no need to manually add a lot of step to do the same thing. I think it is similar to the object-oriented programming concept of class and object. When we create some objects, they can reuse the same function.