What is the usage of the IVisitable interface in Visitor Pattern?

Im really trying to understand the importance of this interface, but beside helping us to write more quickly, the methods in the concrete classes (by only implementing the methods) I just can't find the need to use it.

The definition is this

an abstraction which declares the accept operation. This is the entry point which enables an object to be "visited" by the visitor object. Each object from a collection should implement this abstraction in order to be able to be visited

." Its clear, but still you can manualy write those accept methods in every single class(which is lot of unnecessary work I agree) but still beside that you can get a class to be visitable, without the IVisitable interface...