A 3 piece wheel allows you to change your offset and backspacing with relative ease, since the outer rim and inner rim are two seperate pieces.
With a 2 piece wheel, either the outer and inner wheel are one piece, or the wheel center and inner wheel are one piece... It isnt always as easy to make backspacing or offset changes with 2 piece wheels, but its still possible.
Another advantage of multi piece wheels in general though is that you are usually able to repair parts of them fairly easily.
In my case, I bought a set of 3 piece wheels a while before I had made the decision to mini-tub my car. After I got the wheels, I mini tubbed the car. Now all I have to do is get a new set of inner wheels to make them wider and take advantage of my new found space.