Your cars paint has started to delaminate and peel off. This is not only visually unappealing, but it exposes the steel or aluminium underneath making it vulnerable to rust and corrosion. If a large area of metal is exposed, then the elements are able to deteriorate the structural integrity of the metal making the vehicle unsafe.
The paint has been faded by the sun or scratched by other elements. This is going to make the car look worn and aged and detriment to the resale value of the car. The main goal of a respray for this use is to restore the “factory look” of the vehicle giving it a new literal lease of life, as even the oldest of cars can look showroom ready with a fresh coat of paint.
You may just want to change the colour of your vehicle in order to change the entire look of the car. This is much more of a custom approach so the world is your oyster when it comes to what colour you would like your car to be.
The cost of the respray is going to depend a lot on what your main intentions are with the car. A simpler approach is going to be less intensive with work and time, so it is going to be less costly than painting a car to showroom quality, which takes more time, resources and effort.