Succeeding in health and fitness goals depends a lot on the way you are being guided. Studies have shown a better success rate in people who train with a professional who is certified in a particular fitness specialization. It is more like a sport and the more guidance you get, the better you tend to grow.
You could set your personal goals and decide to start your attempts at weight loss or improved physical health. But chances are, you would be doing it a certain way you know. And it is not necessary that you would be doing things right all the time considering the fact that there would be no one to guide you or point out where you might be going wrong.
A course personal trainer or someone with a personal fitness training certification would be adept at providing the necessary instructions at every step. Right from setting realistic goals to keeping you motivated all throughout, a professional with relevant experience would give you the individual attention you need in a private one on one atmosphere. Apart from that, they would also be better informed regarding the kind of nutrition you would need to keep up with the fitness plan you choose.