Personal trainers help keep you accountable to your goals. Fitness and fat loss is difficult and most people just can't do it alone. A personal trainer helps you sty committed and achieve your goals.
A personal trainer will design for you a fitness program unique to you, your goals and your abilities. That program will make efficient use of your available time, takes into consideration what you can do and what you are capable of doing, so that you don't waste time.
A personal trainer will design for you a program that is safe and will monitor you during the workout so that you do the moves safely without letting you slack or over do it.