I totally agree that this is what should be done first! That is even one of the biggest reasons why we always want access to the production systems of our clients!

However, taking (only) this approach causes you to miss out on any possible functional issues there might be (e.g. to much data being shown or processed by the client(s) ).

Taking a step back and looking at the big picture forces you to look at all possible sides of the problem. Taking a techie approach is just not always the way to go.