Why use gains on an ADC
ADCs typically have a fixed number of bits. If the signal of interest does not cover the full natural range of the ADC input, then some of those bits get &quot;wasted&quot;, reducing the effective resolution of the result.
The analog gain block matches the range of the signal to the range of the ADC, maximizing the effective resolution and reducing the level of quantizing noise relative to the signal.
You could address this by using an ADC that has more bits to begin with, but an analog gain block is usually a lot more cost-effective.