My take is that it's leftover from HPS/MH days. 3000K 80CRI has about 12% blue. 3500K has about 15%. Standard HPS has 6%.
3000/80 is good for flower and even a dedicated veg lamp, though I may catch some flack for that statement.
I'm absolutely sure fan leaves under 3000K are larger than 3500K across many strains. I suspect 3500K produces more secondary branching but it was a casual observation and I can't say for certain but I always felt I was doing more trimming under the 3500K lamp. This might be good for vegging a mother, but with a single topping you will end up with bushes either way from most strains so for vegging a plant in prep for flowering I would still choose 3000K. Higher K lamps are more efficient, so that needs to be taken into consideration but pulling watts out of the red zone also lowers the phototropic efficiency. I'd like to see someone do a veg and show pictures stating "this is what 3000K veg looks like, this is what 3500K looks like, this is what 4000K looks like, etc.". Beyond that the veg K preference seems like a shaky opinion. If someone is working in a height restricted area that might make high K more attractive, but I didn't notice any stretch/height difference between 3000 and 3500, so if 4000+ produces shorter plants I would like to see some show and tell.