Four: It Keeps Your Goals Measurable
Whether your goal is to complete the Tour de France or beat your PR on the local climb, a powermeter will help you get there in a much more solid, straightforward way than training with perceived effort or even heart rate. This is because as your FTP (Functional Threshold Power) increases, so does your performance, and FTP is a number you can see, track, and train for in a concrete, measurable way. Heartrate is another measure of effort, but not a measure of performance.
Five: It Keeps Riding Fun
Ok. We might get some pushback on this one. But we don't care if your idea of "fun" is an epic 12 hour gravel grinder, a Saturday morning ride with the local crushers, or a solo ramble through the countryside - if you're riding stronger, it's going to be more enjoyable. Riding with a power meter might seem to be overkill if you're not planning on racing, but if used in a even the most basic of training plans, it can make all of your spinning easier by building muscles and fitness that allows you to stay fresher longer. Why would you not want that option?