resistors are simple and cheap, but play a very important roll both as a design item and as a device you need to design around.
for the simple LED, the resistor limits the energy passing. this creates a change in the energy and by proper selection, the LED is fed with the exact energy it needs.
put two resistors in series and each will change the energy and in the section between, you can select what you need. typically, this is used as a voltage divider when you want to create a specific voltage. or, one might use a very high resistance so that when there is no power on the circuit, that very high value one will bleed off any stray power or feed tiny bits of power to keep the voltage up at a level.
a long wire has a calculated resistance so you could use a long wire if needed, or you might need to increase power to compensate for the loss by the resistance in the wire
there are many more uses, applications and devices that you have to know in order to calculate power.
electronics is a very exacting science. in 98% of the uses the applications can be awfully sloppy and get the job done. however, the amount on knowledge needed to truly understand a circuit is pretty amazing.
the bottom line is that there are a handful of basic formulas that you need to know and understand in order to make sure things work properly.
for instance, you have an LED that has a 2 volt drop and can work from 10mA to 20mA and you have 5 volt power available. you can calculate the proper resistance based on voltage.
however, the next calculation is power. say that 1 LED is really 25. that resistor value did not change, however the power needed to be delivered to all those LEDs did. an you need to calculate how many watts of power that resistor will need to handle in a second calculation.
resistors are a fundamental building block. you should take the time to really understand their use and application.
