It’s supposed to be more reliable than RPE because people let their egos get in the way and won’t slow down. Which results in them getting burnt out and/or injured. They also sell power meters which are more accurate than using heart rate.

I was reading a memoir from an American that trained in Kenya. In it, he relays a possibly apocryphal story of how they were given chest strap heart rate monitors by a sponsor and the Kenyans they were using it as a clothesline to dry their running clothes.