Hardened steel on aluminum means that the aluminum has to give.
Small flakes at a time.
Facing a bottom bracket on a bicycle after chasing/cleaning the threads.
When working on the head tube (where your steering tube runs through the frame), the process is different, but still ends up with lots of aluminum bits. That process is called facing and reaming, where the facing is done similar to this, but at the same time a different steel bit is reaming the inside of the tube to prepare the surface for a bearing cup.
The purpose of the facing is to make sure that the ends of the tube are parallel with each other. If not, the bearings and system will not work properly.
This is typically only done once in the life of a bicycle. If the manufacturer is good, they’ll do it. If not, your bicycle shop should have done it before selling it to you. Unless it’s a really cheap bike, when it’s just not worth the time to do it.
It’s all good fun and just reinforces for me what an amazing piece of machinery the bicycle is. Especially as cheap as you can get them these days.