The idea behind this is that you can change all instances of a certain class to a new class, so long as both classes belong to the same element.
An example would be two H2 element classes: "H2a" and "H2b", both with completely different styling to them. Let's say your website currently has fifty H2a instances. But what if you wanted to see what your website looked like if they were H2b instances instead? You'd have to either change all the styling in the H2a class to what H2b has (and change it back to H2a when you're done), or you'd have to manually go through each H2a instance and change it to H2b. Both are extremely tedious. A much simpler solution would be an option in the tools somewhere to change all H2a class instances to H2b instances.
This opens up the door for tremendous possibilities. The entire look and feel of your website could be changed in mere seconds compared to hours.