PHP is an object-oriented programming language and we can define classes and functions like any other OOP language. Laravel is a PHP framework and provides great advantages over other PHP frameworks.
Laravel classes can easily be imported anywhere in the application. If you are thinking to create your own classes and functions in Laravel it is equally easy to do.
One of the reasons we may want to create our own classes is to have customized functionality in our Laravel app. Though we can write code in the controllers but its recommended to extract extra functionality from controllers to other classes or functions. This way we can keep controllers clean and more focused on the main task.
Classes can also be used to reuse the code over and over again in your application. For example I create custom classes to generate user notifications and initiate the notification class wherever I want.