Run your fingernail across an unturned rotor and run your fingernail across a freshly turned rotor.
On top of that brake pads bed with rotors junk gets between the two an contaminants and materials get embedded in the rotor so starting fresh is a good thing. You also loose your rust lip.
Is it necessary in some cases no. But for $20 it certainly is not hurting you. I have no desire to go 100k plus miles on a set of rotors. So loosing a bit of material but still being in spec does not bother me in the slightest.
20 years and no cracked rotors or failures so I’d say it works pretty well.
All the O’reilys auto parts around here still have machine shops.
I've never had a cracked rotor and I don't think it makes a difference whether you turn them or not as far as that goes. Rotors either rust away and they start to separate the layers of metal or they get warped, or a pad wears through and grinds into them and grooves them badly.
There is no doubt that a freshly turned or a new rotor is smoother but it's really insignificant. Once you put the new pads on and just a few stops the pads wear and seat into the wear on the rotor and you get plenty of breaking ability.
But like you said, either way works just fine. The O'Reilly's around here also turn rotors and drums and most of the Autozones do too. It is at least $10 a piece now if not 12 to 13.
For a larger vehicle rotors it might be worth it but for the cheap front wheel drive knock-off rotors it's been just about as cheap to buy a new one as it is to turn them for years. Of course, that's not if you let them up sell you to the better brands. But I have never had any problems with the standard house brand rotors in these parts stores.
In the early 2000s I was getting GM front wheel drive rotors for 13.99 a piece.
These were the WearEver brand from Advance Auto Parts.