They can be a useful training aid, but you will need some time using one to know where your heart rate usually is/how you perform. I've had basic ones that just show you HR but don't record - used to use these in time trials just to see how I was feeling. I now use it with my Garmin so it records the data - it's useful for recording fitness progress - e.g. you can directly compare rides and monitor heart rate over particular climbs. It can also be useful if you are not well, or certainly an indicator of illness.
I was on some nerve pain medication last year, and taking it made my heart rate jump about 10 bpm over where it should be - I was maxing out on climbs when the perceived effort was low - I stopped taking the medication !
You don't need one for strava though.
