Ohm's law is mainly used when building electrical devices to determine the amount of resistance needed to establish a certain current with a certain amount of voltage. Without Ohm's law, the circuit may have too much or too little current or voltage.
Resistance and resistors limit the amount of current traveling through a circuit, therefore establishing certain levels of voltage. Most appliances need a certain amount of voltage to properly operate, and Ohm's law helps you maximize the efficiency of the machine by not producing excess voltage. Ohm's law mainly calculates resistance, but it may also be rearranged and used to calculate the voltage or current a given resistance produces.