Only in the aspect that it gives you a number you can look at after your ride, compare to other rides and maybe more solidify your goal in your mind.

However I've used HR and cadence since I got serious about riding for the exercise and health I needed. For that reason I just mentioned above. It helps me to see where I was and what I am now. If you ever decide to get a coach or talk with someone that knows a lot about training on a bike, the past data might be useful to them. I too can't yet justify the cost of power meters. I'd favor Garmin Vector 3's if I could.

If you have a smartphone already, you can try some apps right now. Some have a free version with less features or a free trial period or are just inexpensive. An Wahoo TICKR HEART RATE MONITOR strap and Wahoo BLUE SC SPEED AND CADENCE SENSOR, both bluetooth/Ant+ will leave you positioned well if you decide to try a dedicated device later.

I picked dedicated device in your poll, but that's because I prefer them. You and others might prefer a smartphone with apps. It just does not matter unless there are some specific features or signal reception issues that may be pertinent to you or another individual.

Smartphones generally do better in urban environments with tall buildings. The phone can use cell towers to to augment the gps satellites and find recover faster from a brief loss of signal. A dedicated device might get better GPS service when way out in the middle of nowhere. However if you want someone to know where you are while riding, you are going to have to have a smartphone to provide the necessary link for data communication.

I like Garmin devices, I've used them for hiking, driving, sailing and cycling I've got/had probably over ten different Garmin's. So they are what I'm used to using and dealing with their quirks, which every mfr's have. I've heard Wahoo devices are good. Lezyne SuperGPS is likewise supposed to be good too.

I like the reviews this guy does....... https://www.dcrainmaker.com/product-reviews
His picks are not always mine, but that's because he has different needs. However he writes a thorough review.