It comes down to your goals for using an HRM and what your doc says.
I started using an HRM because I got into my 70s and started having an indescribable problem on some rides - total lack of energy and weird feeling in my chest sometimes, especially riding into headwinds during the 2nd half of some rides. My cardio couldn't find anything wrong with my heart.
By tracking my rides, essentially as suggested by RChung, I found that when I was feeling good I'd go out and get myself to a high HR (130s-140s) for 30-45 minutes. I'd rest at my turnaround point and have no energy left, so the ride home was exhausting. I quickly learned that keeping my HR below 130 on the way out left me plenty of energy for the ride home.
Later I saw some evidence of tachycardia, which resulted in a few bouts of wearing medical-level HRMs for extended periods and a pacemaker plus beta blocker (HR sometimes too slow and sometimes too fast, beta blockers reduce HR). My doc tells me to ride and to let my HR go as high as it goes. Other docs may have different recommendations.