Adding new appliances to an existing electrical circuit without checking total load can lead to nuisance tripping or, in more serious cases, overheating wiring that was never designed to carry the combined current. Each circuit is rated for a maximum current, and calculating total load involves adding up the running current of every appliance likely to operate on that circuit simultaneously, not just the highest single appliance.
Air conditioners and other compressor-based appliances have a startup current noticeably higher than their running current, and this surge should be accounted for when planning circuit capacity, since a circuit that handles running load fine can still trip during simultaneous startup of multiple units. Dedicated circuits, wired directly from the distribution board without sharing with other outlets, are generally recommended for high-draw appliances like air conditioners, geysers, and washing machines, reducing the chance of overload from unrelated devices being plugged into the same circuit.
Older homes and buildings, wired before central air conditioning became common, often have electrical infrastructure that was never designed for the combined load of several modern appliances, making a professional load assessment worthwhile before adding significant new equipment rather than assuming existing wiring will simply cope.