a piston ring holds back a lot of pressure and hence is called a precision item, so precise that it needs to "shape" itself according to the cylinder.
If you notice new rings - they are rough cast at the edges - this rough casting needs another rough casting to slide against in order to make a seal. The other rough casting is the hone marks - aka deglazing.
The hone marks hold oil on the cylinder while providing a bearing surface for the rings to seat and hence seal on every combustion event.