When is the Apple Camera Adapter necessary?

My understanding is that the lightening charging cable won’t transmit data.  The camera adapter will bypass the iOS device’s internal DAC and just pass the unconverted digital signal that then can be sent to a USB DAC. 

I use the camera adapter into an Audioquest Dragonfly Red amp/DAC and realized a significant performance improvement with my headphones as opposed to plugging them directly into my iPad.  BTW, I also tried an aftermarket lightening camera adapter because it had the capability to charge my iPad as I’m listening to music, which the Apple adapter cannot do, and I found the Apple adapter to actually be a little more transparent.  So if sound quality is paramount I’d recommend paying up for the Apple camera adapter for critical listening with a quality headphone or audio system.

Hope this at least somewhat answers your question. 

My understanding is the camera adapter is just an ‘adapter’ and if your DAC or player accepts USB-A, you should be able to transmit content through this connection.

So I did some more digging and I found these pictures on the iFi website. Does this mean that the camera adapter is unnecessary for USB-A DACs, or is this just marketing?

https://ifi-audio.com/a-perfect-match-iphone-6-plus-micro-idsd/

I took a quick look and I think @kalali is correct in that the usb charging cable will transmit data with the main difference seeming to be that the charging cable has a male usb connector and the camera adapter has a female usb connector. There is a usb3 camera adapter that transfers data faster (usb 3.0 vs. 2.0) and has a lightning port too so you can charge your device or provide more power for equipment that needs it (microphones, etc.), but it is a little bulkier due to the added port. That’s all I got.