I think we needed Apple to do that to throw a lot of weight behind the standard so it didn't get stuck in an eternal migration that never ends. Though removing the SD card slot was dumb since USB-C was never an alternative to SD cards.

Even today the desktop PC market is still stuck on USB-A since they have no Apple equivalent to just get things done.

I really don’t understand what SD card reader on MacBook Pro is for.

All small cameras and devices use micro SD. Professional cameras don’t use SD since forever now. Basically I always need an adapter now.

You can use an adapter for a micro SD but you can’t shrink the size of a normal SD card, is my only guess.

I don't think that's right, professional cameras still use full sized SD cards.