Its partly legacy because in the 00s Apple didn't want non-tech savvy people to get frustrated when trying to get their music on to iPods. So when 'docking' you

Its partly legacy because in the 00s Apple didn't want non-tech savvy people to get frustrated when trying to get their music on to iPods. So when 'docking' your iPod, iTunes would automatically open and you'd have your purchased music right there and you can just drag and drop. They got extended to iPhone when it came out. As the actual on device buying process became better over many iPhone generations then the need for a computer to dock to at all became unnecessary.