Qualified trainers can dramatically improve your workouts, helping you to achieve things you never thought possible. You should consider working with a trainer if:
You always do the same workout: People often get stuck doing the same workout simply because they don't know what else to do. A professional trainer will evaluate your strengths and weaknesses, and be full of good ideas to get your workouts back on track.
You are uncertain how to do various exercises: Good form is crucial both for avoiding injury and getting the most out of your workout. If you are unsure how to do various exercises (especially those such as squats, etc, that have potential risks) it's worth having a trainer teach you the right way to do them.
You need motivation: If you often find yourself simply going through the motions, you will find that having someone to work out with will ensure that you show up and work hard. A good workout buddy can do this for you, but not all of us have the ideal training partner available.
You have committed to a major event: If you are taking on a serious challenge, such as a triathlon or marathon, it's worth getting an expert to design a programme for you. This will maximize the effects of your workout time, and ensure that you reach the starting line injury free and ready to race.