But how do you know what 190 represents for you?
The zones represent exertion levels. Zone 2 to (low) Zone 3 you should be able to ride in all day, so long as you eat. Upper Zone 3 you would struggle to ride for more than 2 hours. So I can monitor my zone and go as fast as I can in that zone, without risking over-exertion.
If you've done a max HR test (and my max HR is ~13% higher than it's supposed to be for someone of my age), then you will have the numbers for your zones. So you know where you are in the zone - so you might as well use the zones in you cycling computer.
If you haven't done a max HR test then you have no idea what 190 bpm means to you. The formula for working out max HR is bunkum.
Of course, you can just go by perceived exertion, which people did for years. I'm not confident in my perception of my exertion, though. I'll always go off too fast and then collapse in a heap, puking in a hedge, or fail to complete a ride.
Sometimes I do feel a bit like Froome, staring at his stem all the time, so I do the occasional unplugged ride and I don't wear a HR on my commute, either.
Edit: also, my memory is bad. When I'm out on my bike, it tends to get worse. Zone 2.8 on the cycling computer is easier for me to interpret than 153 BPM, and I don't have to remember my numbers for the zones.