Why deals with the various drivers and/or constraints (internal and/or external) imposed upon a solution.
These are examples of the kinds of questions that you should be asking with respect to the “Why” of the intended solution:
Why is the solution needed (e.g., change in business conditions, new initiatives, legacy system replacement, technology upgrade, etc.)
Which statutory or organizational rules is this solution intended to satisfy (i.e., rules that are drivers for the solution)?
Which statutory or organizational rules does this solution need to satisfy (i.e., rules that are constraints on the solution)?