Yes the is some stuff already in Apple Health, but I for me personally had the need to see not just the zones for single workouts. So with Heart Rate Zones Plus I can see the zones I was in each day, week, last 7 days, month and last 30 day. Also the option to just check time in moderate and vigorous exercise per e.g. week was missing for me. I try to hit the recommendations of 150 moderate minutes and 75 vigorous exercise minutes here - and be able to check that in a fast easy way.

Also I have a coworker who is never actively tracking the workouts in the gym. As I'm using all heart rate measurements, I would still give you an idea of you zones. Also if you run up the stairs Heart Rate Zones Plus might catch that as high zone.

In addition the calculation methods of Apple for the Zones are just "one method" I added a few more options I found in scientific literature of how to estimate, because depening on you fitness and body in general it may be different for you.

Sort of. You can only see your real-time heart rate on your phone when you're doing a cycling workout for some weird reason. Otherwise you have to awkwardly look at your watch if you're walking/running.

I was surprised that the watch is the awkward thing to look at for you, as I have always found a phone to be awkward to look at while working out in any way. Learned a new perspective today.

This doesn't entirely help, but for most (if not all) workouts, you can set alerts for specific heart rate zones. Of course, this is very cumbersome to do per workout on the watch.