Tim, competitive running is all about times, and benchmarks.
Yes, going sub this number of that number is an arbitrary notion, but it's the best way to look at it.
Imagine telling someone next race, "yes, I've improved by 2%", they'll look at you like a nutter,
If you're also musing about how can people casually say they want to drop 5,10,15mins from a pb, because that's a lot, well clearly it depends how slow they are to start with!
If someone runs a half marathon in 3hours, then clearly they can easily drop it by 15mins with more training.
There comes a natural point where we're all talking mere seconds of improvement though....