Hi,
A HRM is best for training............................If you have a goal like improving fitness / just maintaining it.
If you want to improve fitness you might find you never get there with out some sort of guide to effort level.
If you have cycled with a club etc then its possible your mates are the best incentive........just trying to keep up with them
In my youth I was always late for work and my internal pace today is the same, even when I haven't got a HRM is the same.
Some I have found like your modern sportife riders, got the carbon 7.6 kg bike got the jells got the I wanna be there, fail miserably because like all things its Bloody hard work getting fit.
But I would still want to know that I wasn't kidding my self...............that's easy to fall into.
If you have long since given up a goal hard to hit, have already got there etc. Then you don't really need one.
One day I will drift in to turning the pedals slower and fuel up on beers.............If I can ever afford to.................
P.S. If you struggle to get your HR up and don't use BP meds etc, then just try turning the pedals faster for half an hour on the turbo WITh 80 % of max HR at 300 - 350 watts (on the road 5mph faster than your average if that be 15mph then 20 mph) Faster = 95 rpm. (12 stone male, more if over weight ) .
There are times when you cant do it and some will be confused at lack of performance / low HR, it affect all of us, you have an off day, also if you are still unfit / inexperienced with maintaining high effort / high HR then it will be impossible till you get fitter.
If you can't maintain 80 - 86% average of your max for more than an hour then you need to get fitter if that's what you want

Food and hydration are important on the bike too.
Good luck.