Of the many reasons people mention for using a personal trainer, it's usually motivation for training that comes up first.
No matter how much you might enjoy working out, there are times when it is just hard work - that's where a personal trainer comes in. Having someone else to work out with makes it much harder to skip a workout or take it easy if you don't feel in the mood.
Motivation for training comes in many forms, and the good trainer will adapt their technique from client to client to ensure that each one gets the most out of their workouts. Some people will need the sergeant-major type growling in their ear, and pushing them all the way. Others need a more gentle approach - an encouraging word, a quiet nod, a quick smile - while for some, the mere presence of trainer will be all the motivation they need.
A personal trainer is a workout partner who is almost guaranteed to always be there - why? Because you're paying them! Unlike your friends, they won't wimp out because they're not in the mood, or not show up because they have to work late. By the same token, because you are paying for the session, you are also more likely to show up. Most people report that the reason they quit working out is that they get fed up, or simply feel that they're not making progress. Having a personal trainer right there with you the whole time means that you always have someone to encourage you, make sure you're doing it right, and keep you working hard even when the going gets tough.
So if you are someone who knows that they will need help from time to time simply to keep working out, then it's worth considering using the services of a personal trainer. Let's see how easy it is for you to quit when someone else is watching your every move!