Advantages of a maintenance plan
Preventative measures are better then the cure. Most people let their cars collect so much dirt that by the time a first-time detail comes around, it is quite literally impossible to restore it to the original. Just like a regular check up at the doctor's office keeps you from getting sick, and may prevent serious disease, a regular detail on a automobile will keep it in the like new. Dirt is not allowed to sit for an extended period of time and therefore does not become a permanent part of the car.
Just like anything else in life, prevention is the key. Regularly cleaning your vehicle's interior and exterior will naturally extend the life of the car, and naturally, will increase its value if you decide to trade it in.