What is an individualized exercise prescription for?

Personalization. It's a guide that's tailored to meet a person's specific exercise/fitness goals, accounting for possible underlying medical ailments. It will often include very detail plans, including daily exercise routines, the weights involved, number of repetitions, rest times, time and intensity of any associated cardiovascular exercise.
Different strokes. Exercise prescriptions are useful when a patient may be at risk for developing a disease such as hyperlipidemia or for injury rehabilitation. In the first case the provider may prescribe exercise as a first line treatment before putting the patient on lipid lowering pharmacotherapy. Treatment is individualized per patient to ensure they are able to perform the given exercises.