The main benefit is that everything else also uses USB-C, so you can standardise on one type of cable.

What is special about high-speed USB-C cables that sees them not work with low-speed USB-C devices (i.e. the iPhone 15 non-Pro model)?

High speed USB-C cables work fine with low speed devices. Just at lower speed.

So why is the stated main benefit lost?

The benefit is that you can use any USB-C cable for charging. You can also use any USB-C cable for data transfer at low speed, which is what the iPhone supports.

With iPhone Pro, you can transfer at low-speed with any cable or high speed with USB3 cable. But most people would rather carry a cheap, light, 6 ft charging cable than heavy, expensive, 3ft for the rare case of transferring data.

But why is USB-C not a benefit of the iPhone 15, per the discussion? It has a USB-C connector, only limited by its USB2 implementation.

No one in this thread has made that claim.

The contextual parent asked, in response to a comment lamenting the they don't want to pay for the Pro iPhone model to get high USB speeds, why you need high speed USB-C outside of some niche cases related to video (where you are already bound to want the Pro model for the better camera). The response was that the main benefit (of high speed USB-C) is that you can standardize on USB-C cables.

But it is not clear why high speed USB-C cables are not compatible with the iPhone 15, which has USB-C but does not support high USB speeds. If it were, then the purported main benefit would also be found on that device, leaving the question to be unanswered and the attempt at a response nonsensical.

Fast backups: 30 seconds instead of 5 minutes.