We are only supplying 100 ohm, 1k ohm, 10k ohm, 100k ohm, 1M ohm and 10M ohm. If you notice, all of them are actually 1 to the power of 'x'. This value of transistor is actually are good for prototyping. For example you have three LED with GREEN, RED and ORANGE in colour and you power up it using 100 ohm resistor as figure below.
After you power ON the circuit, you noticed that each brightness of the LED is not the same. RED LED is the most brightness compare to others and you did want all the LED has the same brightness. After you made some I=V/R calculation, you notice that if all the LED to have the same brightness, GREEN LED must use 100 ohm resistor, RED LED must use 500 ohm resistor and ORANGE LED must use 250 ohm resistor. Figure below show how you can parallel or series up resistor to get this 100 ohm, 500 ohm and 250 ohm value using value '1' resistor.