Not specifically an Apple product, but just to give a real-world example of where this kind of thing matters:
At work I was cutting open a standard 3-prong computer AC power cable (the thick black ones with the D connector on the other end) to put a plug on something hardwired. When I stripped back the wires, it was immediately obvious something was wrong with it.
After finding a proper cable for the job, we investigated a bit. Turns out that this cable (and a couple others that had come from the same batch of sketchy Chinese products), which was stamped UL listed and rated for 10A--so should have been at least 16ga stranded copper inside--was not only made from copper-plated steel wire (it was magnetic!), but was probably 22ga.
These cables had shipped with products that had extremely low current draw, so they were going to (barely) work with the item they were shipped with. But had you thrown one in a box with 50 other similar cables--like we did!--and then plugged it into something the label indicated it was usable with, it would either have not functioned due to low voltage or overheated, melted, and set your house on fire.
That was the moment I realized on a visceral level that counterfeit products weren't just a "well, it probably works the same" issue, but something that could have very serious safety impacts.
Because "not tested" means who knows what's actually inside.