To each their own. That's just my personal methodology.
Here's my reasoning; my .375 Jaguar shoots lights out to 330. We have steel at my local range at 330, 480, and 600. I can consistently ring a 6" steel plate at 330, which shouldn't be too difficult for any caliber. At 480, even though there's no reason I should see a difference, I can't maintain regular hits on an 8" plate. With my 6.5cm, 600 is a piece of cake down to 4" plates, so I know it's not form or ability. The .375 just seems to start wandering a bit somewhere between 330 and 480. So my self imposed limit for that particular rifle is 350. If I hadn't tested at those ranges, I would have assumed that being good at 100 and 300 would mathematically let me stretch to 500 maybe. But with that setup it doesn't work that far out.
I'm not a good enough shooter to understand what's happening there to cause the issue. So, with any load I make and intend to use for longer ranges, I want to test and verify that there's not something goofy afoot before I try it on a critter. It's not worth wounding something because I didn't do my due diligence to verify I was indeed good to that range, and instead got lazy and just relied on the math.