Because some hardware manufacturers are dumb flaxs and you need that to use regular HDDs in their flax. *Can'tSayThisOnTV*ing nonsense, but its dead useful if you need one.
No, those come attached to regular hard drives from the factory. They put extra engineering money into making a special adapter to put in each and every laptop because they didn't want to just put regular SATA connectors right on the motherboard. This was long after SATA was an established standard, mind you.
UGH I HATE THAT CRAP JUST USE THE STANDARDS Dangit