It is basic interview prep. At base, it shows that A) you did the homework and prepared for the interview and B) you probably have some aptitude that will help getting through training. Why fight it? It's just not that hard.
Understanding aircraft systems is important because it helps/reinforces the checklist and procedures, especially emergency procedures. Some actions require steps done in a specific order. If you understand how the system is built and works, it can help you understand why things have to be done in a certain order.
For emergencies, it can help you understand if the problem you experienced is a minor issue (write it up for maintenance) or if it is the beginning of something more serious and you need to start looking for a divert.
Maybe I'm just old school and a product of the military, but I have had to learn basic engineering systems (hydraulic/fuel/pneumatic/electrical...etc) for every ship and airplane I've operated in the last 20 years. It is something that separates the professionals from the recreation crowd. I'm not knocking recreational flyers/boaters. Just saying that as a professional, you should have a higher level of understanding. And that higher level of understanding has indeed helped me in both flying and ship driving.
In my opinion, If as a professional, you don't hold yourself to a higher standard and want to be just a button pusher, then you might as well get replaced by a computer.