My guess is the DCIM mount is for things like photo kiosks designed to plug any photo device in to print pictures. It would kinda suck to have a smartphone that couldn’t talk to such a system, but a modern one could easily get around that now, and digital cameras are far less prevalent.

As for mounting it as mass storage, usually there is a notification you tap into that gives you options. Most of the time it defaults to charge only and you have to opt in to full storage access to prevent plugging in to some random “charging station” and have it copy data to or from your phone.