People train for a variety of reasons, it could be to lose weight, change body shape, prepare for a wedding or holiday, rehab from illness or injury, run a marathon, pre- or post-pregnancy; the list is pretty extensive. Hiring a personal trainer means clients get regular, dedicated time with their trainer, and are completely focussed on their own personal health and fitness objectives. 

Motivation is a big issue with exercise, having a personal trainer means you can't shirk out of your exercise session in favour of the sofa or a long lie. You know they'll be knocking at your door or be in the park at a certain time to get you moving, and you won't be able to turn them away. Still, you'll need to put the effort in yourself - a personal trainer isn't a magic formula and you won't change into a lean, mean fighting machine overnight!