Power meter – nice to have.
A power meter can lead to improved training by giving you a serious insight into how much power you produce when cycling. Your heart rate can fluctuate depending on how you feel on a given day, but your wattage does not change much. Therefore, you can keep an eye on how much power you produce so you can keep the sufficient pace throughout the whole route.
When you learn how to monitor your wattage numbers, you can as an example keep an eye on how much you could push yourself on the hills, knowing that there is still enough fuel left in the tank for winning that final town-line sprint.