Why are resistors important?
Resistances are the most important and fundamental passive elements. there must be at least one resistance in each electronic circuit.
The resistors are mainly used to limit the current. but it is also used in many other occasions, such as reducing voltage (according to the voltage divider rule). it is very useful in low power circuits, as when we use any module with arduino, most modules work at 3.3 v, but arduino can provide 5 v. thus, we use two resistors to convert 5v to 3.3v to avoid the 5v supply, where 3.3v is needed. we also use this technique of voltage divider in the fan speed controllers, the light can be adjusted using a potentiometer (variable resistor, so that we can adjust any resistance by turning a knob).
we can use resistors to test loads in any test circuit. For example, you have designed a mobile phone charger circuit. Now you want to test the charger. So, what will you connect to test if the charger is good or not? are you going to connect a mobile phone to test it? If the circuit is not perfect, your mobile phone will be damaged, there is also a risk of explosion. in this case, you must use a load resistor. it is better to destroy a resistance than a whole cell phone.
The resistors are also used as fuse. yes fuse. The resistors have a nominal power such as 0.25 W, 0.5 W, etc. If you insert a low resistance resistor (10 ohms) in the circuit, it will act as a fuse. In case of short circuit or over current, the resistor will blow up the rest of the circuit will be saved.
Resistors are also used to discharge capacitors. if a circuit is capacitive, you can still receive a shock even after disconnecting the mains, because the capacitors are charged. to discharge these capacitors, high resistance resistors (1 m ohm) are connected to the capacitors to discharge the capacitors.
These are just a few basic operations. The resistors are used in various other applications, such as the operational amplifier, and much more.