The decibel (dB) is an expression often used in electrical and acoustic measurements. The decibel is a number that represents a ratio of two values of a quantity such as voltage. It is actually a logarithmic ratio whose main purpose is to scale a large measurement range down to a much smaller and more useable range. The decibel is not a unit of measure such as feet, inches, or pounds. The decibel is a comparison between two values. The form of the decibel relationship for voltage is:
dB = 20 x log(V1/V2)
Where 20 is a constant, V1 is one voltage, V2 is the other voltage, and log is logarithm base 10.
dBm (sometimes dBmW or decibel-milliwatts) is unit of level used to indicate that a power ratio is expressed in decibels (dB) with reference to one milliwatt (mW). It is used in radio, microwave and fiber-optical communication networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form compared to dBW, which is referenced to one watt (1000 mW).
Since it is referenced to the watt, it is an absolute unit, used when measuring absolute power. By comparison, the decibel (dB) is a dimensionless unit, used for quantifying the ratio between two values, such as signal-to-noise ratio. The dBm is also dimensionless but since it compares to a fixed reference value the dBm rating is an absolute one.
In audio and telephony, dBm is typically referenced relative to a 600-ohm impedance, while in radio-frequency work dBm is typically referenced relative to a 50-ohm impedance
Mainly based on two considerations
1. Represent very large or very small numbers, simplifying the numbers, for example: 1000 times gain, receiver sensitivity 0.001mW, which representation of gain=30dB, -30dBm receiver sensitivity are much simple .
2. It can turn the multiplication and division into addition and subtraction, by checking the number tables to handle complex calculations, great benefits to pre-computer era.