If you already have an AC unit that is working, there may be reasons to replace it before it breaks. The primary reason for early replacement is to increase efficiency and reduce your carbon footprint.
Is your air conditioning system in good shape? If it is older than 8-10 years it is going to be less efficient. Since AC systems in 2020 can be 60% more efficient than 10 years ago, by replacing aging units you will save on operating costs. A unit 15 years or older is not only less efficient but likely to run into expensive repairs as parts wear out. Also if it old and phased out refrigerant R-22 in your system, it will be almost too difficult to service and repair, as the chlorine based gas is being replaced by r-410a chlorine free product and other eco-friendly refrigerants in the residential industry.