http://www.processing.org/reference/this.html. " To call this method inside PImage itself one could simply write filter() or could more explicity write this.filter(). It's not incorrect to say this.filter() but it is not necessary as this is always implied." There is a use of using this. in code but I can't think of one right now..

Fietspomp

Two common usage of this coming to mind:
- Initialization in constructors of classes: it can help to distinguish from class fields (variables) and constructor's parameters, if they have the same name:

1. public SomeClass(int angle, String message) {
2.   this.angle = angle;
3.   this.message = message;
4. }

This is the official way, I prefer to prefix my field names. Others prefer to prefix the parameter names, but they will appear in the JavaDoc, if generated.
- Just to pass a reference of the current class as a parameter to a method. In Processing, it is often used to pass a reference to the underlying PApplet to the constructor of a library:
SuperLibrary sl = new SuperLibrary(this, oneParam, another);