Pro Riders using heart rate monitors in this day and age ?

I have a noticed quite a few pro riders using heart rate monitors as I see their chest straps. I thought the day and age of heart rate monitoring was gone , with the advent of power meters and watts per kilo ? Does anyone know why they are still using these units ? I think the pros know when they are in the red ? I think they know when they are going to "pop" and can simply watch their wattage on their head units. So if anyone has any insight , much obliged.
Yes, power measures the output and is an indicator of external physical load. Hr and perceived exertion measure (or proxy) the physiological cost of the output, and indicate internal load.

During a block of training or racing, one can quite easily observe progress of fatigue by a depressed hr response at a given power or effort, or potential onset of illness if the hr response is higher than expected, etc.
I know heartrate is obviously still being used to gauge a rider's level of fitness/form. When you're in good shape, you might be able to push a bit bigger numbers, but a (lower) heart rate during an effort will be at least as important a factor to map a rider's physical state.
Otoh, bit worrying when you get the data of a rider on screen, and it just says "Heartrate: 0"...
My old Garmin chest strap HRM used to crap out like that from time to time; probably just slipped on the chest and isn't directly over the heart for a few moments. My wrist one now does it all the time too.

The stage 1 ITT of the 2015 Tour was in hot weather, and Contador said afterwards that he was well within his power capabilities, but his heart rate was waaaay up, so he decided not to push the watts. Heart rate is a key variable, and easily tracked, so it would be unwise to not at least have an idea of how it's going.