Why can we use 'this' as an instance method parameter?

What is a receiver parameter in Java ? Java 8 Language Specification talks about this.

The JLS gives a hint:

Either way, the receiver parameter exists solely to allow the type of the represented object to be denoted in source code, so that the type may be annotated.

These two methods are equivalent:

class Test {
    void m1() { }
    void m2(Test this) { }
}

However the latter allows you to add annotations:

void m2(@MyAnnotation Test this) { }
//where MyAnnotation can be defined like this for example:
@Target(ElementType.TYPE_USE) @interface MyAnnotation {}

Receiver parameters allow to pass arguments and extract additional information from these arguments. The only purpose of writing the receiver explicitly is to make it possible to annotate the receiver’s type. Now, if you implement the AnnotatedElement interface, you could call the getAnnotation() method of your class to get an annotation type. For more information, you can read this.