There are many reasons to have your car properly detailed.  Below are just samples of those reasons:
•    Health – Clean interiors keep you breathing fresh air and less dust while driving
•    Appearance – It’s been said successful people drive clean cars.  Care about the image you portray by having a properly detailed car while driving others around.
•    Pride in Ownership – A freshly detailed car no doubt looks better than a dirty mess of a car.  It doesn’t matter what you drive, it can look better than other, higher dollar cars on the road if properly maintained.
•    Longevity – By maintaining the condition of the car, you can continually drive a like-new car for years to come.  It’s best to start from day one, but you can protect a car starting at any time to keep it in good condition.
•    Save money – Whether you are selling a car or turning in a lease rental, you can add value to the car by having it detailed and looking its best.  No one wants to buy a filthy car.  Sell it faster and for more money, or get more for your trade in by giving the car a thorough detail before turning it in!

A good regimen to keeping a car looking its best all year long is something along these lines:
•    Month 1 – Reconditioning Detail inside and out – this gives the car a fresh clean start to maintain without having to “cover-up” or “mask” defects.  It is easier to maintain a clean car rather than a dirty one!
•    Month 3 – Waxing for protection – washing will be easier on a freshly waxed car, fallout will not stick to the paint easily, and will maintain that smooth/slick feel.
•    Month 5 – Complete Detail inside and out – exterior wash and wax with interior protection added to prevent fading
•    Month 7 – One step or two step machine polishing to remove defects/swirl marks
•    Month 9 – Complete Detail inside and out
•    Month 11 – Waxing for protection

Couple of notes on maintaining a car:
•    A car needs to be washed once a week, or when heavily soiled, in order to help prevent airborne contaminants from bonding to the paint, maintain a clean appearance, and keep interiors vacuumed and clean as possible 
•    There is no miracle wax that will last forever; don’t be fooled by advertisements!
•    Waxes and sealants have their place.  Generally stated, sealants will last longer, but carnauba waxes will look better.
•    The longer stains/dirt sits, the tougher they are to remove/reduce.  Act immediately as 3 year old grime will not come out!