ADC Reference volatge

What is the purpose of using reference voltage in ADC?

A ADC quantizes a range of input voltages into monotonic numeric values. The lower limit and upper limit is usually relative to 0V and another voltage level, which is the reference voltage, so an ADC reding of 0000 might be 0V and 1023 might be 5.000V for a 10bit ADC with a reference voltage of 5.000V. Such an ADC is said to be ratiometric with respect to the reference voltage, or for an ADC reading of n,
Vin = 5.000*n/1023