Remember hearing your maths teacher say that?
There’s a good reason for it; It gives you the opportunity to demonstrate how you solved the problem. You would be marked on the working out, and not just the final answer.
It’s the same with design work, if you can’t describe how (and more importantly why) things are done a particular way, then the client should (quite rightly) think of it as just a guess.
Being able to do this successfully is probably the most important skill to learn in client services, because without this, you risk a client reject or dismiss good ideas, simply because they can’t see how you worked out the answer.