There not required to know it for PPL. Its just that alot of them find it interesting how the system works. It usually comes up when looking at intercept procedures.
The question of how a fast jet is going to intercept a 90knt spam can is usually the topic. BTW I said to the students if you get something fast like that coming near you more than once select 121.5 and give a MAYDAY, callsign and position and say you believe you are being intercepted standing by for instructions in UK airspace.
Apparently how they do that is need to know as well, unfortunately the pilots that might be intercepted are deemed not needing to know.