Suppose that you have a part of a transformation that you will like to use in another transformation. A quick way to do that would be to copy the set of steps and paste them into the other transformation, and then perform some modifications, for example, changing the names of the fields accordingly.
Now you realize that you need it in a third place. You do that again: copy, paste, and modify.
What if you notice that there was a bug in that part of the transformation? Or maybe you'd like to optimize something there? You would need to do that in three different places! This inconvenience is one of the reasons why you might like to move those steps to a common place - a subtransformation.
In this ...