You don't get good results from any device. You get good results from executing sound basic training principles of consistency, frequency, progressive overload with recovery as needed, specificity and individualisation of your training and development needs.
Most half decent training plans and basic monitoring tools (a watch, RPE, HR and even power meter used in a really basic manner) will get people some way towards executing these principles, e.g. it's very rare that someone I give a 2-3 month training plan to and who executes it does not improve, however such plans sacrifice some level of load management optimisation, specificity and individualisation.
Power meters (good ones at least) and the data they produce provide you with objectivity in assessing the training you are actually doing v. what you think you are doing. Neither RPE or HR can do that.
I mean far more than monitoring your work rate at any particular moment but right though to considering what you are doing in a more global sense. How what you are doing now (or previously, or this week/month etc) fits in with and impacts your season and even your entire athletic career.
Power data also helps one to better understand their current and historical physiological capabilities and its relationship with and response to your training, your physical attributes (e.g. aerodynamics), the specific demands of your races or goal events, and can help assessment of some riding/racing skills/execution, which leads to individualising and optimising your training and development program to suit your specific needs.
As a communications and logging tool, the objective power data balances the subjective feelings about how you are going. Both matter and it's more useful when subjective and objective are assessed together.
And interestingly, and somewhat in opposition to what many seem to think, power meters can actually provide you with a lot of freedom in the way you go about your training since once you recognise what's actually important you realise there are many ways to skin the training cat. Applying good training principles does not automatically imply overly regimented training.
To use power meters wisely and to a reasonable proportion of their potential for performance improvement requires an investment on your part to learn how to understand and apply the data.
Or you could just use it as a fancy speedo, effort monitor and ride logger. If that's all you intend to do though, I'd save your money and just follow a half decent plan and keep things fun.