It's a small thing, testing every bulb, and I do it on every piece of restored lighting before it leaves. It sounds minor. I think the small things are where care shows.
Here's what it means in practice. Before a fixture ships, I put power to it and make sure it lights, every socket, every switch, every function it's supposed to have. If it's a three-way, I check all three settings. If it's got a pull-chain, I pull it. If there are multiple sockets, each one gets a working bulb in the test. I'm not just confirming the wiring I did; I'm confirming the whole fixture does what it should, the way you'll expect it to when you hang it. I rewired the thing, so I know the wiring's sound, but knowing and confirming are two different things, and I'd rather confirm. A fixture can be wired right and still have a bad socket, a worn switch, or a fault that only shows when current runs through it. Testing catches that here, on my bench, instead of at your ceiling after you've done the work of hanging it. When you get a fixture from me, it's not "should work." It worked, here, before it left, and I know because I plugged it in and watched it.
Salvager's Note: Every fixture that ships from here was powered up and checked, every socket and switch, before it went in the box. So it's not "should work"; it lit on the bench. That's a different promise from a fixture wired and boxed on faith.