Knowing the RTT seen by actual user traffic is useful because, for example, it lets you observe if that traffic is encountering excessive queuing. It's hard to know this by other means; for example you may think you can just ping the end systems, but these days ICMP is often filtered, and even it it wasn't, the test probes may be queued differently due to weighted fair queuing, or even take a different path due to equal-cost-multipath routing. In the end, the only true measure is what the traffic itself experiences.
Now, whether it is necessary for operators to know if user traffic is seeing excessive queuing is open to debate. But the fact is they do currently use this as one network health measure.
How though, because surely it includes the time outside the observers network? And if they only want to monitor performance inside their network nothing so them from using an out of band system, or even adding their own timing footers to packets.
