All machines require maintenance in order to perform efficiently and avoid unplanned breakdowns. Cars require oil changes, coffee makers and ovens must be cleaned. Your Air Conditioner is no different.
The heat exchangers (coils) for your outside and inside units must be cleaned in order to efficiently remove the maximum amount of heat from your home. The condensation produced by your system must be piped out of your home and those pipes become clogged if they are not maintained. No system is perfectly sealed so the refrigerant levels must be maintained. All of the components in your system will eventually wear out and it is cheaper (and more convenient) to replace small components during planned maintenance as opposed to unplanned maintenance.