Some people know how to exercise and they even know how to do the exercises correctly, but they like having a trainer around for support and supervision. If you're lifting very heavy weights or need someone to help with partner-type exercises, working with a trainer might be a good choice for you. He can spot you during workouts and help you come up with a good training plan for your goals.
If you'd like to exercise at home but either don't have a lot of equipment or aren't sure how to use what you have, in-home personal training is an excellent choice. A trainer can show you exactly how to use what you have to get the best workout for you or they can bring equipment with them to give you a great workout. They can also make recommendations for equipment that will help you reach your goals.