Not as much a newbie question anymore buster. I bet lots of younger pro-mechanics don't know about it either, since the last time a flat tappet cam came in a new engine was probably over 20 years ago now.
And in addition to what syk said, and more importantly in fact, it's all about the camshaft.
The reason for not idling when the engine is fired up for the first time is that the cam lobes, which are arguably the most highly stressed part of an engine, do not get pressurized oil fed to them. Only the cam bearings get that luxury.
The cam lobes and lifter bases are only oiled by "splash" lubrication. More of a combination of splash, drip and windage/vapor lubrication actually.
The only thing that could be considered pressurized oil delivery is the excess oil that squirts out the sides of the cam bearings, some of which finds it's way to the lobes.
Some of the oil that makes it up to the top of the engine makes it to the ports above the cam as it's draining it's way back to the oil pan. That drip goes straight to the cam lobes.
Since this and the other things are the only way to get oil to the cam lobes, and the initial start-up is the most critical time of parts bedding in, or mating up to their counterparts, we're trying to get as much oil to the cam as possible during the initial few critical minutes while those parts get to know each other while literally sliding and scuffing themselves silly together.
This is basically why you also never mix and match lifters and cams once they're broken in. If you remove the lifters, they need to go back into the same holes they came out of to get the best chance of finding a good mating surface to slide on.
All this is why, during initial startup, you don't want to crank it on the starter for extended periods, let it idle while you fiddle with carburetor and distributor settings, and stuff like that.
If you see an oil or gas leak, you shut if off. If you feel you want to adjust the timing, do so quickly. If you need to adjust points, shut it off. If you need to add more fluid, either do it at the higher speed, or shut it off. If it looks like you're having a cooling issue (temps over 220) shut it off until you find out what's going on.
It's important to make sure the ignition is working and timed at least close (you can tweak it while running at 2500 rpm to at least get it working), and that fuel is in the carburetor.
And that's why you don't idle the engine for the first 15 minutes. Or if you're like me, 30 minutes just to be safe because I don't want to do the job twice!
The speed can be varied, which does help the rings and things seat in more better, and will vary the temperature slightly as well. You can run down to 2000 rpm, or up to 3500 rpm. But not less than 2000 for at least the duration of the break-in.
As mentioned, I like to give everything and everyone a break and multiple heating and cooling cycles to get parts to work together better. So will often shut it down after the first 15 minutes or so. I have run it the full half hour before, because it was fine, the mufflers were installed, and I had nowhere else more important to be at the moment.
But shutting it down to let it cool off for awhile is not a bad thing.
The piston rings, pistons, bearings, and other things can actually handle idling down, but the camshaft can't.
So that's why you don't idle it...
Paul