I agree with you to some extent: when they’re controlling all the variables (Glowforge makes the hardware, the software, and sells standardized raw materials), it should be possible to accurately model what the end result will be. When I take a picture with my camera phone, it looks great 95% of the time, even though the raw signal from the sensor and the raw data to the display need all kinds of complex adjustments to look right. So it should be possible in the simple case to make an engrave on Proofgrade with full-auto settings come out resembling the source image without having to take it through Photoshop adjustments first.
That said, there are a lot of variables involved, and some of this is artistic choice. I don’t expect a fairly dark and muddy image to print well on a black and white laser printer, and I’m also not expecting fully automagic results to be amazing. You’re always going to get better output if you make some strategic adjustments to the image to compensate for the fact that you’re burning it into a piece of wood with concentrated light beams.
The tracing and layering and nodes is probably more than necessary if you just want to engrave a photo. As palmercr explained, you can get away with throwing a raw photo at it, but it probably won’t look great, so the next smallest thing you can do is to tweak the brightness and contrast a bit. That’s not super technical, and you can find some examples on the forum of the kinds of tweaks people do to get their photo engraves coming out nicer. In short, subtle differences in shades of grey are not your friend. A really “blown out” look on the screen translates better to lasering.