What's the point of the CB1?

I bought a BT Pi 1.2 and TFT50 a while ago, but never used it because they arent compatible with each other. Now I'm looking at getting the upgraded BT Pi2 and to run klipper and octoprint on my E3 V2, or possibly the Manta E3EZ. The existence of the CB1 is confusing me.

If the BT Pi1 and Pi2 exists, what is the function or purpose of the BT Pi4B with CB1/CB2?

And if mounting the CB1/CB2 to some of the manta mother boards is effectively like adding a BT Pi 1 or 2, and all the mounting holes are the same, why not just use their regular Pi 1 or 2?

I guess I understand it's function if it's just to save some space when used with a motherboard. But it still doesn't explain why there is an adapter just to effectively turn a CB1/2 into Pi1/2?

The more I read the more confused I get, lol. Someone please advise. Thanks!