When designing a PCB, part of the design process is making sure the PCB works correctly. At the very least, someone does some functional testing. But what happens if problems are found? A more detailed look at the design must be performed. And in fact, the best way to end up with a working design is to check the performance during the design process.
So if you have to choose between simulating or measuring your boards, which would you choose? The best answer is to make time for both. Don’t choose: simulate your cake and measure it, too.
Most people would be tempted to say just measure the board, because that will always be the closest to reality. But that is not always the case, especially with the fast busses used on PCB designs today. Edge rates are so fast that measuring the actual system is not practical, because you would really have to measure inside of the chip to get an accurate view of the performance. Measurements need to be taken on test boards or with special test fixtures, which may not represent the actual system. This is where simulation is indispensable, as it can allow you to look in places the probes can’t touch.