One of the main reasons people benefit and have success with personal trainers is that they need motivation. Having set meeting times, a program tailored to you, and an overall schedule is a huge benefit and makes the 'job' of working out less taxing. Certified personal trainers provide structure and acceptability, as well as help you develop a lifestyle that encourages health rather than just a 'lose it quick' styled plan.
This is a major reason to get one, especially if you have any chronic health conditions, pre-existing injuries or training goals like running a marathon, etc. For example, I have a connective tissue disease, therefore I understand a little better how difficult and strenuous it can be to do some tasks, which means I might be a better fit for people experiencing these issues. Unfortunately weight gain and sometimes loss of muscle tone is common in those with chronic conditions, and even though they desire a level of fitness and health, it's hard to achieve it - especially if you're worried about injuring yourself. A good trainer will work with you and will even work with your health care provider to help you plan a safe, efficient program that is made with you and your abilities in mind.
We all want to workout for two weeks and meet our goals, but it doesn't really work like that; especially if you're working on your own. Personal Trainers help you focus on results and can help you stop doing inefficient workouts. A personal trainers has a plan and will help you get maximum results in a minimum time - in comparison with home fitness.
Improve Technical Skills
During college/schooling personal trainers have to take a course over Sports Medicine (among other helpful classes), which is why they can help with athletics. The right personal trainer can (and will) help you improve your skills by showing training techniques that are specific to your sport. The trainer will incorporate skills training into your new program so you can improve your overall strength and endurance as well as agility and mental focus.
New To Exercise
This sounds silly to some, but some people truly are 'beginners' when it comes to working out. A personal trainer acts as a fitness coach and can drastically help in the prevention of any injuries. It's all about teaching confidence and knowledge.
Personal Trainers aren't exactly nutritionists, BUT we do have extensive knowledge about clean eating, foods to eat, foods not to eat, foods that work together to burn fat, etc. If you don't have the time to sit down and read tons of confusing articles and make a meal plan, a personal trainer can be a huge solution.
The most frustrating thing of all. Sure, you're in pretty decent shape, but you want to push it a little further. If you're stuck in a routine and want to break out of a rut, maybe build more muscle or tone a little more, then a personal trainer may be the solution. The programs that personal trainers learn implement techniques that keep the body 'guessing', no day is quite the same, making the body burn more and build more.
Go It Alone
You just want to learn the facets of designing your own routines so you don't need to use a personal trainer for a long period. I don't recommend seeing a trainer from the YMCA, but rather a free-lance trainer. They'll likely be more willing to show you the ropes. If they're a good personal trainer, they'll work hard to teach you the basics of building and modifying a fitness program to achieve max results.
I've said this already, but I'll say it again - safety. It's one of the most important aspects of working out. A personal trainer watches your form and helps your perfect it as well as monitoring your vitals. This all helps them give your feedback about your limits, strengths, and weaknesses. Most of us tend to ignore some of the more subtle signals, I know I do, and we end up injuring ourselves and knocking ourselves off of our fitness plans because we're laid up in bed.
Work Out At Home
Maybe you want to workout at home, but don't know how. Many personal trainers (specifically those free-lancers I've discussed) will make house calls. This will help bring fitness into your living room! Sometimes it'll cost a little more, but at least you can learn in a place that you feel comfortable.
If you join a gym specifically because of the equipment, you often get complimentary sessions with a trainer (usually one or two). This means you can ask what you want to know and ask what you need to do to get fit. This helps you develop your own flow.
You're Your Own Motivator
Some people just don't need anyone else motivating them other than themselves. If you've got your fitness routine down, your nutrition down, and you're succeeding and still feeling motivated, you don't really need a personal trainer.
The internet has everything. And youtube is great for fitness videos, plus there are tons of great websites that help teach techniques and how to make your plan - granted you sometimes have to pay to access these things, it's money well spent. Especially when comparing the $10 E-book fitness program VS the 1 year gym membership + personal trainer fee's.
You like learning about nutrition and you have the time to do your research. You already have a good fitness program and good techniques, all you need now is a little time put into learning what to eat. It's not so hard, especially with blogs from personal trainers (wink, wink) and pinterest!