analog to digital converter as voltmeter
If you connect a 5V ADC to 120VAC, you will likely have a rather violent puff of smoke. The circuitry of a digital voltmeter is for scaling and protection: it scales a wide variety of voltages in ranges to fit a single range of voltages that is most optimal for the single ADC internal to the voltmeter. This circuitry is also responsible for making sure that no harm comes to the operator or device in the event that the device is connecting to high potentials or otherwise misused.

A typical ADC used in a DMM might have a resolution of 16 - 20+ bits and an input voltage range of zero to a few volts. Clearly, such a converter cannot directly convert hundreds of volts, nor can it accurately convert very small voltages directly. Thus, additional circuitry is required.